Federal law prohibits the transfer of the handgun to you unless it is for certain reasons. There is a provision for inheritance.
Check out: http://www.atf.gov/publications/down...f-i-5300-2.pdf
Check your NJ laws but I'm sure there is a trustworthy adult of 21 years age who could hold it for you until you are of age. If your Poppop lives in a different state it would have to be transferred legally to someone of age in your state.