Clothes... simply wear something you have that wont stick out like a sore thumb in the environment where you'll play. There is NO mandatory need for special gear unless a scenario requires it.
The comments you'll get if you stick out will be 'nice target, do you like pain?'
Age to play at; already written about. If you follow the rules instead of trying to take shortcuts, or lying, or whining about it, you'll be fine.
The only problem with underage players are those who lie to play or to get guns. It's a matter of legality, not maturity. There are ways to deal with maturity problems on the field; most include banning across the province or further until a clue sinks in. Note that lying is far from being mature.
Concern about guns being around; pick something that lasts, pick an AEG first. I strongly suggest you look at any Marui product. They have a proven track record. Or at least read a LOT about the different models and go see what folks use at games before spending. What people actually use is a far better indication than what is written about.
Ball is in your court.