World Gen is very Bipolar, I've had one weak demon that I could kill with one punch, it had 200 kills to its name...
And a Dragon who could kick my ass barely had any.
So I've come to the conclusion it has little to do with fighting capabilty, also, while Dwarves start off old I've seen Human civs in older versions who were all children in year 3 of world gen.
Prehaps you should research before you act like a douchebag.
Quick check to the legends mode usually shows that all races, for the first 100 years or so, have mysteriously well aged people. Often, an 80 year old team of dwarves will found the first dwarven civ on the first year, just as elves and humans and goblins will do. Beasts themselves, however, need to grow. I believe that only Dragons need to reach adulthood, but they start at age one if I'm correct. Of course, Megabeasts don't have their ages listed in Legends, so that's very speculative. MBs are immortal in worldgen anyways. You have more to worry about if you, say, gen only one year and then play for a while. The no-longer-invincible infant dragon has a good chance of dieing.