I'm nearing my 20s, and like everyone mentioned here, people usually write characters with similar ages or at least near their own age range. Personally , I like writing characters near my age group because I feel like I can make them more fleshed out since I'd be more or less aware of the kind of issues and troubles they may or may not deal with.
I used to like making teenage characters for my comics , but then given the setting of a lot of my stories, having a bunch of teens as the central characters sometimes makes it feel implausible.
My character's ages are :
Laine : 23
Altair: 28
Mira: 27
Heike: 36
They're pretty much around their early 20s to early 40s. With the main character perhaps being the youngest in the cast. Oddly enough, my characters are waaaaaaaayyyyy older than I am. orz