It has long been a staple among the Mac faithful that Macs may cost more than equivalent Windows-based PCs, but (along with many other advantages) they retain their utility longer. I’m certainly guilty of such statements, and I’ve backed them up over the years by keeping my SE/30 (upgraded from an SE in 1991) in useful service until 2001, at which point I replaced it with a Performa 6400 that was at least five years old.

That said, hardware longevity — how long the actual hardware continues to function using the software of its era — is being undermined by the need to maintain software compatibility, particularly with networked software. For many years, an elderly Mac could remain useful even in the face of new and incompatible system updates because computers were relatively isolated from one another; as long as file formats remained compatible, older machines maintained their utility. The first hint that networked software was going to become important came from Web browsers, older versions of which weren’t always able to load Web sites using the latest Web design techniques.

But Web browser compatibility is nothing compared to the compatibility issues Apple has raised with iCloud, which works only with Mac OS X 10.7.2 Lion and iOS 5. Suddenly, older Macs and iOS devices that aren’t compatible with Lion and iOS 5 have been excluded from life in the cloud, regardless of how well they run other software and even modern Web browsers. In short, the effective life of hardware is now determined by Apple’s corporate fiat, rather than organically as the Macintosh industry gradually shifts away from supporting older machines.

This got me thinking. When some new version of Mac OS X or iOS comes out, we always report on the hardware with which it’s compatible, but we’ve never brought all the different operating systems together. To do that, I pulled out MacTracker, which provides introduction and discontinuation dates, and used EveryMac’s Ultimate Mac Sort Tool to determine which Macs were made obsolete by each subsequent version of Mac OS X.

The aim here is to figure out just how long you will likely be able to continue installing operating system upgrades (and thus software that requires those OS versions) after you purchase a Mac or iOS device. In particular, I’m interested in the minimum lifespan — how long a particular device would be supported by Apple if you bought near the end of that model’s lifespan.

First, though, to address an early comment, it is true that Apple continues to support the previous version of Mac OS X (though not iOS) with security updates. So, during the reign of Mac OS X 10.7 Lion, Apple will release security updates for 10.6 Snow Leopard, though not for 10.5 Leopard or anything earlier. While this is a welcome policy, I don’t see it changing the functional lifespan of a Mac, since you don’t get your work done with security updates, you get it done with a functioning operating system and supported applications.

Mac OS X -- Here’s what I found, starting with 10.4 Tiger, which was released in April 2005. Tiger supported all PowerPC G3-, G4-, and G5-based Macs, and was the first version of Mac OS X to run on Intel-based Macs. There’s little utility in going back any earlier, since previous versions of Mac OS X (with 10.0 released in March 2001) also supported all PowerPC G3-based Macs with the lone exception of the original PowerBook G3.

Tiger was superseded by 10.5 Leopard in October 2007, when Apple started to drop support for installing new operating system updates on some older Macs. In particular, Leopard swept off the shelf all PowerPC G3-based Macs and slower PowerPC G4-based Macs whose clock speed was less than 867 MHz. Most of those Macs had been discontinued by October 2003, except for a lone 800 MHz iBook G4, which held on until April 2004. So Leopard supported all Mac models introduced as far back as 7 years earlier, but the last Mac sold that couldn’t run Leopard was taken off the market just 3.5 years before Leopard shipped.

Next up was 10.6 Snow Leopard, which Apple released in August 2009. With Snow Leopard, Apple drew a line in the sand at the Intel transition, eliminating all PowerPC-based Macs. Looking back in time, the iMac was the first Intel-based Mac in January 2006 and the Power Mac G5 was the last of Apple’s product line to make the jump to Intel, sold as a new product until August 2006. (The PowerPC-based Xserve remained available until November 2006, but it wasn’t aimed at the consumer market.) That sets Snow Leopard’s backwards compatibility to as little as 3 years, a year less than Leopard’s. This is understandable given the enormity of the architectural change.

With 10.7 Lion, which came out in July 2011, Apple consigned a few early Intel-based Macs to the dump heap of history. To be specific, Lion requires an Intel Core 2 Duo processor or faster, which left a number of models out in the cold because they relied on the Intel Core Solo or Core Duo processor. The last of these to go was the Core Duo-based Mac mini, in August 2007, putting Lion’s backwards compatibility at just under 4 years. The first Core 2 Duo systems were sold in September 2006, which adds almost another year to those Macs’ upgradable lifetime.

iOS -- What about iOS? The first version of iOS to drop support for earlier models was iOS 4, which appeared in June 2010, and wouldn’t run on the original iPhone and iPod touch from 2007. Apple stopped making those devices after a single year of production in June 2008 and September 2008, respectively, giving iOS 4 a backwards compatibility of 24 to 27 months.

iOS 5, released in October 2011, also tossed an iPhone and iPod touch over the side: the iPhone 3G and the second-generation iPod touch. (To be fully accurate, these devices actually first bit the dust with the release of iOS 4.3 in March 2011, but that was a relatively minor update and including it would muddy the analysis significantly.) The iPhone 3G survived for 2 years, remaining for sale as a low-cost alternative even after Apple introduced the iPhone 3GS in June 2009. It was eventually discontinued in June 2010 when the iPhone 4 came out.

Similarly, the second-generation iPod touch was introduced in September 2008, and while the 16 and 32 GB versions were pulled from sale a year later in 2009, the 8 GB version held on for 2 years before Apple stopped offering it in September 2010.

Technically speaking, that gives iOS 5 a backwards compatibility of only 15 or 16 months, to the last date the iPhone 3G was on sale as a new product. But it also marks the first time Apple introduced a new product while continuing to sell its direct predecessor. On those grounds, you could argue that the real backwards compatibility of iOS 5 is 27 or 28 months, for the iPhone and iPod touch, respectively.

(Michael DeGusta worked up a fascinating chart comparing iOS and Android OS upgradability by phone model up until June 2010. He chose to look at the span of time from a phone’s introduction to three years after release — less for phones released in the last three years, of course. He depicts across that period how long a phone was for sale, how long updates were available, and how far behind a phone was compared to the current version of the operating system.)

Support Summary -- To summarize, then, it seems safe to say that if you buy a new Mac now, it’s a good bet that Apple will support it with new software releases for 4 to 5 years, depending on when you buy in a given model’s lifetime. Snow Leopard cut the time to a low of 3 years for some outlying models, but the desire to focus on Intel-based Macs easily explains that.

Things become more complex with iOS. If you’re buying the current generation of iPhone, you’ll have 2 to 3 years of support from Apple — the longer period if you buy a new model immediately — before you’re left by the wayside. The lower end of the range syncs up with the length of most mobile phone contracts.

However, this will fall down with new purchases of the iPhone 3GS, which Apple is now giving away for free to anyone who will sign a two-year contract with AT&T. It seems entirely likely that the iPhone 3GS won’t survive the next revision of iOS, which means that iOS 6 could be a non-starter for phones that were sold just before the release of iOS 6. (Apple might signal iOS 6 by discontinuing the iPhone 3GS several months ahead to avoid causing some degree of buyer’s remorse.)

The iPod touch seemed to follow the same pattern as the iPhone for the first few generations, but when Apple released the iPhone 4S, there was no associated fifth-generation iPod touch, meaning that the fourth-generation iPod touch, introduced in September 2010, is still current (albeit in both black and white versions now). That may mean that today’s iPod touch will have a much longer lifespan, if we assume it tracks with the iPhone 4, perhaps even approaching the 4-year upgradability mark that nearly all Macs have enjoyed.

It’s also hard to know what will happen with the iPad. The original iPad was released in January 2010 and replaced by the current iPad 2 in March 2011, but both can run iOS 5. The original iPad and iPhone 4 use the same processor — see below — while the iPad 2 and iPhone 4S use a later version. That processor difference could be the trigger that starts the clock on the last possible update. The original iPad and iPhone 4, despite introduction dates offset by several months, may both be thrown under the train with iOS 7. That’s certainly no sooner than 2 years from now. If this wild speculation is on target, that would give the iPad a Mac-like longevity of about 4 years.

Dark Clouds Rising -- The wild card in all of this is iCloud, which requires iOS 5 and Lion’s 10.7.2 release. It’s not so much that iCloud is itself uninterested in the past, since Lion works on all Macs sold in the last 4 years or so. The problem is iOS 5, and the way Apple is keeping obsolete products for sale at lower price points. The iPhone 3G can’t run iOS 5 and thus can’t participate in iCloud, but you could have bought an iPhone 3G as recently as 15 months ago. Thus, you may be able to connect a 4-year-old Mac to iCloud, but not an iPhone that’s less than 2 years old and still under contract. Since the entire point of iCloud is to route data among your many devices, this discrepancy is troubling lots of people.

It may seem that iCloud (via iOS 5) is a bit like Snow Leopard, in that it’s making arbitrary decisions about who’s in and who’s out. But with Snow Leopard, those decisions were based on an obvious technical difference — PowerPC versus Intel processors. With iOS 5, though, there’s no such distinction. It runs on the iPhone 3GS and the third-generation iPod touch, which reportedly use the Samsung S5L8920 CPU, whereas the iPhone 4 and original iPad use Apple’s A4 chip, and the iPhone 4S and iPad 2 rely on the A5. Perhaps iOS 5’s system requirements are based purely on overall performance, which isn’t something that users can see or that we can estimate based on known specs. Certainly, iOS 4 on an iPhone 3G was nearly unusable for a while, until Apple released an update with performance tweaks, and even then it wasn’t snappy.

If we’re lucky, the real boon of iCloud will be Apple moving additional processing into the network, as Siri does, enabling iOS devices to maintain their utility longer than in the past. Apple may be trying to push the iPad and iPod touch longevity into the 4-year range enjoyed by Macs. If you consider the iPhone 3GS and 4S as intermediate versions of the iPhone, it’s possible we could see iPhones lasting somewhat longer as well.

As long as Apple continues to sell vast quantities of iOS devices to new buyers, everyone is happy. But should Apple feel the need for more upgrade revenue at any point, it seems clear that the company can arbitrarily declare the obsolescence of an entire generation of devices and potentially enforce that obsolescence with some sort of networked service that works only on the most recent devices.