I am 35 and have been biting my nails as long as I can remember. Its a habit. It is was it is. But I look at it this way, I don't drink or do drugs, so if I only bite my nails...there isnt much complaining going on :)
Joanie46 seems to have the right idea. I bit mine forever until I was put on medication for anxiety condition. It is an OCD thing really and nerves. Only problem is that it is ugly. I hated it, but couldn't control it either as all nail biters will tell you.
Nervous habit - one of the worst because it can cause infection on fingers as well as transfer germs from hands to mouth. I bit my nails for years. Now I have great nails, but twirl my hair like there's no tomorrow. Nervous habits are hard to break, and are usually traded off for still another nervous habit - bouncing a foot, licking of lips, picking at cubicles - etc. Prone individuals will always find an outlet for nervous energy. It's relatively normal.