Well you dont need a credit card to buy a sub, for cash you can go buy one in any shop so its not at all hard to get online. Either way paid sub or not, you are never going to be able to stop immature/irresponsible players on any online service or game.
I think the only way that Live attempts to stop under-age players by getting onto live is by checking the D.O.B on the windows.net thingy that links your Hotmail, MSN, Live etc together. But it is easy to bypass by entering a false D.O.B into that...
My younger brother had this problem when he was setting his Xbox up for Live as he is 17, but it wont let you on withought a parents CC info for reasons of buying credits online, but a simple change on the .net and some time to let it update was easy to let him on.
The only way to eliminate that is to play in private matches with groups of friends. Or mute button