>>33764138Part that.
Part their personality, as May's "brash, vaguely tomboyish girl" archetype is regularly applied to 15-17 year olds in other shit.
Part Dawn looking slightly more like an actual person that could exist, despite having blue hair
Part conditioning through CynthiaxDawn stuff
Part May seeming older because you've known her longer (Hell, she might have even been older than you at one time)
Part May seeming older because she has a younger sibling, and that younger sibling being a terrifyingly competent 6 year old (or however old he is)