Different ages if a great idea. however I would limit it on these 3 ages. It should be smoother. I mean it\'s not like \"hey kid! today you grow 18, so you\'re a man.\"
Growing up is a progress with many shades.
rather use some formula like:
wise = exp * age
learn = exp / age
strenght = abs(30-age)
ok, ok, these formulas are only to show the concept. don\'t tell me that a 90 years old man would be twice as strong as a 30 years old

to display the sprite in game: 10-16, 16-25, 25-40, 40-60, >60
another question is: do you grow older in game? and is there a way to get youth back? like some water of live?