primary mac user here, though i have a vista box, a fedora box in a server role, and a dual boot xp/fedora desktop. i deal in tech support with a variety of OS's, find it comes in handy to have some around at home.
i agree that you're more likely to run into a situation where a combination of your computer and a given software title "just won't work" on a mac when dealing with an age gap, but much of that is due to the changes that apple's implemented to try and move in a new direction.
If they'd had a bigger share of the market 5-10 years ago, i don't think there would have been quite such a steep cliff. that being said, that issue does arise on windows as well.
10 years prior to the release of vista, win95 was the current OS, try running a win95 app on vista, or an app designed for vista on win95.
10 years prior to now, win98 (the first release) has been out for a year or so. trying to update a pc you bought with win95 to vista would be a long slow, likely fruitless process. with some significant hardware upgrades you 'might' be able to have a usable xp installation on an early win98 pc, but even that's unlikely.
the move to 64 bit OS's and apps will make things even more interesting for compatibility. basically. if you want things to work, use recent hardware, recent software, and try not to do anything too outside spec with it, or expect to be supporting yourself (which isn't impossible, but not something john q public's gonna do)