I must admit, though I played vanilla DF for years, my only generated worlds are limited to "pocket" and "smaller" sizes and I always start them at year 05. They end typically in their 20th-30th year due how lagged with large quantity of items DF becomes or how incomplete automation of tasks is done. Micromanaging after a while is too bothersome. Animals and dyes are a particular pain of mine. I am contemplating not to deal with any animals, besides cats and dogs in all my future plays. I slowly grew only my embark locations. This gives more time, when invasion arrives in embark and makes them more realistic. It was suggested to me by other players btw. It has its limits. Largest size of embark becomes bugged with annoying map's inaccessible zone bug after 5 years or so, which is making digging and construction impossible. I never got issue getting all civs I desired at year 5, though it seems Dark Towers and Dwarven Fortresses are easily wiped out for some reason. I get dropped with kings way too frequently, while 100+ Goblins disappear in mass pit every year. Elves and Humans are toughest survivalists on every map, but then I never get a single siege from them and they rarely are in conflict with almost non-existent Goblins.
I could imagine large maps with implicit goal of adventuring, after initial fortress embark to get adamantine stuff. However because of my experiences with largest size embarks, I have little doubt they are bugged similarly to hell as well. Toady never set up from devoted players any team of DF-testers, who would go through all modes and variations to check everything is ok and functional, short, medium and long term. He is not even programming all features. Loose ends everywhere.
Toady and some other players seem to prefer stiffened old maps with 500+ years of history on them. For the story line I guess. I find 05 year embarks the most dynamic. I can go to near-by fortress just month later in Adventure Mode and find there new stuff, new items, new weapons, new configurations of rooms, new shops and so on, which wasn't there earlier. Not to mention the refreshed and renewed stockpiles. The only thing that beats down on me during visits is the shifting architecture of Dwarven Fortresses. Some parts are becoming temporarily unavailable/inaccessible. A bug I hope. Like the endless animal spawns and lagged to hell caves and Dark Towers. Unfortunately playing DF, even on good CPU, takes literally months and months at a time. At least for me. So, you can not explore all possible options and situations, as you would like. I guess 100+ years old embarks is good for players looking for a particular scenario to play, which is impossible to get at year 05. Plus lag for large population of historic animals. Majorly historic animals. Not historic persons. If you start map with adventurer instead of embark, then you have in a year of travels around 10000+ pages (screen size 1600x1200 more or less) of encounter reports to go through. Majorly reporting about escaping animals, who avoided combat with you. Game promotes those animals to recruits and they still flee in panic at slightest detection of your sneaking in shadows. Is there a second guess why so much lag on old maps and why experienced players advise so strongly against playing those "matured" maps?