Most of my characters are between 21-30, with a few being outside that for certain situations [i](1x superhero school RP where I was a 17yr old girl, 1x Post Apocalyptic RP where I was over 30 man)[/i]. I gauge that to be because I in fact....... 28.
So therefore it's easier for me to play an older character.

I assume it's the same for a 15 year old. How can a 15 year old understand the emotions and feelings a fully grown adult will experience, and then understand them enough to create a character and roleplay that adult like character?