When I was 12 I had my own real firearms. I did not get to tote them about at will though.
Without personally knowing the child in question there is no easy answer. If you want something they can keep in their room and use at will get them a Nerf gun. They can use it in house, shoot their friends, and so on.
I suspect you've got your kid bending your ear explaining they need to buy that ICS and at the end of the day it's up to you to decide. If they attend organized games supervised by adults at an established venue, then the answer is maybe.
If they want it so the can tuck it in a gym bag and use it in the park, etc. then chances are high this will end badly.
Anything in between the two will probably end badly also.
Airsoft, where nothing is hurt but feelings.