Pretty much the same as for otherswhats a good set up for a 12 year old
3000psi aluminium air, halo hopper, goggles that are comfy and gun of choice
It will depend on where and what type of games will be played
If playing punter sites only such as delta force then minimize the gear bought and star with decent goggles
If allowed to play with your own gun at a punter site then something basic and mechanical
If going for walkons, scenarios, tournament style then a basic electronic gun
Depending on how slight and weak or big and tough a 12 year old then that may dictate the gun and air, however a modernish electronic gun and a 3000psi aluminum should not be too be too heavy for most players