I'm a youngster, 19 years old.
When I first got serious about writing my own stories (right around age 11), all my 11-year-old friends were writing stories about 16 and 18 year olds. I read their stories and thought, "well gosh, these characters don't act like any 18 year olds I know", so I made a rule that my characters could only be my age or max three years older. I finally dissolved the rule a few years later, but I would bet you money that well over half of my characters are in between the ages of 12 and 15. Even 16 and 17 year old characters are kind of few and far between for me, ha.
That said, in my practice comic the main cast consists of people aged 15, 27, 33, 35, 53, and 58. That's probably the most diverse age group in any of my stories, ha. I have another couple of comics on the stove, most of which have casts in the 12-16 or 16-20 age range.