I would say it's up to the player. If you were playing a mayfly-kin, I'm pretty sure you could convince plot to let you live longer than 24 hours :lol:
In general, except for creatures like tortoise-kin, I think most Wylderkin wouldn't live as long as humans. They have a poorer diet and poorer living conditions, which would significantly decrease their life expectancy. Worse comes to worse, go with human life expectancy divided by 2 (otherwise known as Sarr age)
I found out tarantulas can live to be 40, that seemed like a reasonable age to me so I stuck with it. Finding out the age of your character then is easy, you can do some simple algebra to find out the equivalent of your age when done in the base of your race.
Say you were 20, and your character had a life expectancy of 40. Human life expectancy is about 80.
20/80=x/40
20 times 40= 800
divided by 80= 10
so your character would be 10 years old.
As far as the age of adulthood goes, that's also up to the player. A race could judge adulthood on deeds, not age for example.