my response before my computer crashed...
well, the best way to determine if he is too young to play is to call the field and ask.....
you will definitely need to have a parent go with you and stay at the filed though.
before he ever airs up and fires that sucker though, you will want to drill saftey into his sponge-like brain. teach him about muzzle control, why he should have a barrel bag on it at all times, even if not loaded and most importantly, why he should never, EVER take his mask off on the field.
now, to get him used to being shot....
you can do what i'm doing with my little brother (12) who wants to play... get him to put a mask and a protector for his 'boys' on, the give him a double tap to center mass (getting hit in the chest hurts the least, but dial your marker down to around 220) this may not work for you, because your brother may not be the type who would take that well
you could practice doing 'runthroughs' while under fire if you have a big enough/safe enough backyard. let him shoot at you for a few runs, then tell him it's 'his turn' hit him once or twice, max. and have your marker dialed down.
why dial it down? expectation of it to hurt alot is what's gonna freak him out. by providing a shock, and a bit of a welt, he will get used to being 'hit', when the shock factor is gone, he will probably enjoy actually playing so much he won't notice the extra 30-60fps extra when on the field