I've come to believe that numerical age is meaningless. I've met forty years olds that look like they're sixty and sixty year olds that look like they are forty. But, it's not just about looks, it's also about behavior. How many young adult books have you read where the protagonist, often only fifteen or sixteen, face adult challenges and by meeting that challenge may seem more adult like than another fifteen year old.
I think responsibility matures fictional and real people, sometimes in a positive way, sometimes negative. But also a person's notions about what they can and can't do because of their age can make them older or seem younger. I know fifty years olds that still ice skate because they love it and I know twenty four year olds that won't dare lace up a pair of boots because they believe they're too old for it.
I understand that sometimes physical limitations and stress can play a role in a person's perceived age as well.
I guess I find it interesting how different and almost malleable our "age" can be via our attitude, nutrition, physical health, and external factors. When we create our characters, they're not just seventeen year old girls or twelve year old boys that fit in these perfect compartments set aside for people those age, we've got to think about them from a multi-factorial view point to really determine what their true age is.