My DS aged 2 started biting his nails in December, and has not stopped since.
I did all the wrong things at first- painted his nails with Stop & Grow, which helped only for a day or so, told him off and constantly pulled his hands out of his mounth in an effort to stop the biting.
Worse still, I'm convinced that the fighting between myself and DH (also in December - we nearly split up) caused it, so seeing him biting his nails is a constant reminder of the worst December of my life and the guilt I feel for causing my precious little one such stress makes me hate myself.
After talking to a psycologist and paed, we have since January completely ignored the habit, and try at all times to keep his hands busy with toys, clay and when we're in the car encouraging him to hold on to his carseat. The biting is a lot less now, but DS still bites like crazy every so often (esp after being told off).
I need to hear from someone who went through this with their child - how long did the problem last and how did it eventually end.