I know it's quite fashionable of late to claim that "old school" is either purely subjective or indefinable in the manner of pornography ("I know it when I see it."), but I simply don't buy these claims. Most of the confusion arises, in my experience, from people failing to distinguish two uses of the term "old school" that, while valid in certain contexts, have no bearing on what grognards like me mean when we use the term "old school." These two uses are as follows:
1. "Old School" = "Old": This the most subjective usage of the term. Gamers who began with, say AD&D 2e consider it "old school" because it was released almost 20 years ago, just as I am sure some 3e gamers will call their edition "old school" in a few years (if they're not already doing so). I frankly consider this usage laughable, as it's both narcissistic and, more importantly, utterly devoid of "scholastic" elements. That is, this usage makes old school simply a turn of phrase to mean "not current" and pays no heed whatsoever to the idea that the "old school" games encompass a philosophy or approach to gaming. This "old school" isn't a school at all.
2. "Old School" = "Adhering to Game Canon": This is another popular usage and one with which I have limited sympathy, even if I don't think it's deserving of the name. It holds that "old school" means "consonant with the earliest editions of game X," whether mechanically or (more likely) based on setting elements, etc. Under this definition, a carefully researched 3e book that holds true to details set down in earlier editions about, say, elementals is "old school," whereas 4e isn't since it creates a new canon that rejects and contradicts the earlier one. This usage treats "old school" as brand identification rather than anything scholastic. It's all about ensuring that, if Gygax said demons do X and 3e says demons also do X, then, because Gygax and 3e are in accord, 3e counts as "old school."
Neither of these usages is what I or most grognards mean when they talk about "old school." For us, it is a school of thought we're talking about. It's a philosophy of game design and game play that emphasizes loose rules, the sovereign authority of the referee, and player skill over notions of "balance," "story," or "fun." Granted, all these emphases are fuzzy around the edges and there's room for quibbling over whether, say, RuneQuest qualifies as an old school game or not, but that's a far cry from saying there's no such thing as an old school. Likewise, despite the fuzziness, the old school still possesses enough rigidity to clearly exclude certain games from its honor roll. No one who uses this third and primary definition of "old school" would ever say, for instance, that Ars Magica is an old school game, despite its being over 20 years old. That's because old school does have a very clear meaning in most cases.
Now, I don't object to equivocal uses of the term "old school." After all, I use the term "grognard" to refer to old school roleplayers rather than wargamers, which is its original definition. Nevertheless, I think it's important to realize that the old school to which I most frequently refer and the one that's currently generating renewed interest among some gamers is not primarily characterized either by its age or by its adherence to hoary canon so much as by a spirit that has kinship with the spirit of the early days of the hobby, when rules were suggestions, referees made rulings based on them, and players, not characters, were whose skills were tested in play. That's the old school. The rest is often either simple preference or mere nostalgia.