It seems that nail biting crops up at this age.
My daughter started doing this at this age. LOTS of kids I know (friends kids) also started doing this at this age.
Our Pediatrician also echoed the same thing.
For some kids it's just a stress release thing, or out of boredom. I asked my daughter once "why" she did that.. and she just said "I don't know... I like to do it when I'm bored...or about to go to sleep." I think it helped to 'relax' her.
But each child is different.
My daughter is now 6 years old, her habit started at 2, and then now she still does it, but very infrequently. Nothing we did stopped it per say.
As she got older per her maturity, we just explained to her that biting nails is a way that people get sick... because germs hide under the nails and it's not very clean to put them in your mouth. For my girl, these logical explanations work with her and she totally understands. And no, it did not make her hung-up on it or on 'germs.'
Her Dad, my Husband, also bites his nails... and HE tends to catch a cold after doing so. He admits it, and explained this to my daughter as well. The nails/hands are in fact how germs are spread after all.
In any case, kids bite nails for a variety of reasons. Each being different. And yes, some kids just bite nails and then it goes on and on as adults.
Another thing is, once my girl got her cuticle all sore, because she tried to bite off a hang-nail... so it turned red and sore... and she told us. We didn't scold or punish...but it was the perfect "opportunity" for explaining another reason why it's not too good of an idea to bite her nails. So we put a band-aid on it, and explained we don't want it to get infected.
You can try and dig deeper, see if he has other "stress" problems.. .but really, sometimes there is just no reason and over-micro-analyzing it can just go overboard, beyond the usual normal developmental and cognitive changes they go through in normalcy.
I never tried the nutritional aspect... the poster made some good points and some children do lack certain things. For my girl, she is healthy and a good eater and takes good/natural stuff for supplements as needed, not on a daily basis though because she has a well rounded diet.
In any case, here is a link on it:
http://parentcenter.babycenter.com/0_nail-biting-why-it-h...
All the best,
Susan