Nail biting is a habit in both children and adults. It is usually a sign of anxiety or an impulse control disorder. As long as the nail bed remains intact, nail biting doesn't affect growth of nails. Research shows that nail biting has pros in that it causes the nail to grow even faster than in people who don't bit their nails.
However the cons outweigh the advantage. It increases risk of infection as it encourages the spread of germs from the nails to the mouth. Finger nails can also be utterly disfigured due to prolonged nail biting. Saliva may redden the skin around the finger nail.

The most common way to avoid nail biting is applying a clear bitter tasting nail Polish on your nails. This will discourage you from biting your nails. One should also avoid factors that trigger nail biting e.g. boredom. Find other ways to manage anxiety. Also one should try trimming their nails neatly or have them well manicured.

if one is so concerned about it, you should consult your doctor about it and he / she can give you suggestions on how to stop nail biting.