There's quite a few examples of worlds that completely wear down over longer histories. By two or even ten thousand years, all civilized races can either be extinct or barely clinging to life in caves. This typically stems from having too many megabeasts for them to handle, and currently being unable to repair their sites after they sustain damage during beast rampages and warefare. That's why most towns visited in older worlds have so many abandoned shops and destroyed buildings. It's not that nobody wants to live there, it's that they are currently unable to renovate those areas.
On the other hand, young worlds just don't have as much going on in them. Yes, there's more megabeasts to fight, but most of the time they haven't had time to distinguish themselves or acquire enough loot or notoriety to justify going after them. There's also a lot less vampire, werebeasts, and necromancers/custom interaction users, meaning it's more difficult to find such individuals for whatever purpose you desire. It's more likely to find more diverse night trolls earlier on though, that manner of generated monster is relatively easy to kill and the majority tend not to survive world gen, but the ones that survive and proliferate through longer histories make worthy opponents.
There is one other quirk of longer histories, highly skilled immortals. Elves and vanilla goblins don't die of old age, and that gives them an eternity to hone their combat skills. This leads to a large number of elite warriors among such races, especially elves.
Also, there's a gimmick you can pull with very long histories. The longer history runs, the more likely all dwarves/playable races will die out. However, as long as a nation of playable creatures existed at one point, you can embark as that nation in fortress mode with seven spontaneously generated individuals, with two guaranteed(hard-coded) migrant waves of similar such individuals. You will never receive any migrants, traders, or sieges, which some people really really enjoy. Right now, there seems to be a quirk where one of the founding seven will declare themselves the nation's ruler, appoint a hammerer if none exists, and issue a mandate far too early for you to handle in such a case. That's not a bug, but it is something you have to deal with right now.
Personally, I prefer worlds that aren't too young but not too old. I try to gen it to a thousand years, give any dragons from the beginning time to reach adult size. More recently, I've taken to genning to anywhere from 300 to 700 years, and that works as well.
Although this leads to partially abandoned cities with lower population densities than hamlets, it also leads to a lot more diversity in those populations, and I frequently see equal parts elf, dwarf, and goblin mixed in among human settlements by that time. There are many night creatures for me to hunt and destroy, and whatever megabeasts and titans live that long have done enough damage that slaying them is something I can brag about without ego.