Yes, I remember when my wrist would show appropriate age but I would have my hand bones showing a younger age.  Now being 27 we figure I just inherited my grandmothers small hands.  If anything he probably will be a great basketball player or football receiver with those big hands.  If you are really concerned, ask your doctor if they have a BMD test/using the Prodigy Advance DXA System.  This actually scans the whole body and gives more accurate measurement of your age.