Yep. ABV. That's it. Lower ABV beers (1.040-1.050 SG) are ready to drink in about 2-4 weeks. Higher gravity beers (1.065 and up SG) taste nothing but the alcohol the first 2-4 weeks, and get alot better after about 6 months or so. Beer lasts alot longer than homebrewers realize if kept out of sunlight and at a good 50-55 degrees temperature. Even Lower ABV beers can last years and still taste good. I've heard this, though, never experienced it. Beer doesn't last at my house. Even big lagers that need to age don't last longer than about 3 months over here. It's a shame, really.
Mead and wine, on the other hand, benefit from ageing more than beer. Beer flavors are very assertive? Upfront? Not subtle? Mead and wine flavors are more complex? More subtle? As alcohol flavors mellow in mead/wine, other things that were in the background become noticeable. Beer flavors are very pronounced, and only mellow and round out with time. So people don't generally say, man, that IPA really came into its own after 2 years. The hops flavor that is on display in an IPA is there from the very beginning, and doesn't need the alcohol to tone down to be tasted.