DD did this at 2 and HV suggested discouraging by gently moving hand away from mouth and saying 'DD don't do that' Was very tedious but worked after a few weeks. DS started biting at about 3 and has proved more persistant (is now 3.9 and still biting though at least he seems to have stopped biting his toe nails to the quick)shock] I usually say 'DS don't bite your nails' Trouble he can reason with me and will try and argue the toss - 'But I liker biting them Mummy'
DD (2.9) has bitten hers for a few months now, which horrified me as DH has disfigured hands due to this.
I couldn't ignore it so have gone for the gently discouraging route, telling her not to bite her nails and moving her hands if necessary. I did notice that if she doesn't watch TV then she doesn't bite her nails - everything else she does involves her hands so she can't then do any biting at the same time. So a joint approach of not doing activities where she can bite her nails and a gentle discouragement is working for me (and saving my sanity)
ds is 18 months and has just started nail biting (isn't that a bit young?!) thinking about trying gentle discouragement, but concerned drawing attention to it in any way will only act as encouragement, iyswim.
Ds does it too - and started at 18 months, now 23 mo. He hated having his nails cut so at least thats easier. Currently I ignore, but not sure if thats the right thing to do. its just a case of picking my battles!