At this stage, your child’s brain is developing fast and he is busy learning multiple things at home and school. He starts becoming active in his social life around this age. Your child develops the capacity to make associations between things based on his experiences and these patterns of associations are stored in the nerve wires of the brain, and he's likely to develop nervous habits. For example, if he faces a difficult mathematical problem, your child might get nervous, and to mitigate the tension he might start biting his nails till he is able to solve the problem.
What can you do
Motivate your child to discard the habit by making him aware of it. Discuss with your child why the habit is bad and work together on getting rid of it. Identify the trigger or condition that gives rise to an association to be made. Maintain a habit diary and adjust the child’s environment to remove the causes to negative habits. For example, why does the child feel nervous doing a mathematical problem? Is he under any undue pressure to perform well? Work on finding an alternative response when faced with the condition.
The moment you see your child veering towards his old habit, make a detour and offer him the alternative. For example, Instead of biting his nails, he can tighten his fist to reduce the tension. Let him practice these alternatives till the new association is firmly established.