G.B.
My DD started about 5, and we tried everything. (she also liked to pick scabs). We even used bite stop nail polish,band aides and even threatened to take away her BELOVED pet chickens.
She is 12 now and doesnt bite her nails any more. About a year ago or more, I started giving her daily minerals. In particular, vit b complex, vit E, iodine, cod liver oil. And a magnesium/calcium combo every other to third day. I don't know if this all helped, or, if it was just coincidence and she is "growing out" of it. Oh and one other thing she eats more raw veggies now- because now she eats my salads.
If nothing else,you could try the b complex and cod liver oil, both will calm nerves and contribute to a sense of well being.
Gail