I would do only slight things like, if you're playing a MUCH younger character than you, wear a color such as a bright pink or yellow or white to make you appear younger, wear your hair differently, wear a skirt, etc. Older, wear pants or a straight skirt, etc. That's only for girls, tho (the skirts part)
if you want to show castability with age, just try tiny things like that, but never wear a costume.
some things are meant to be...