What's the difference between the gamer zones for xBox Live? Whats the age to join xBox Live?

I'm not so wise when it comes to this xBox Live stuff...What are the gamer zones for? Can you switch them after you create your account? Also, how old do you have to be to use xBox Live? Will you not get as much if you're younger?|||ya make your account over 18 because you won't be allowed to download certain content that is rated Mature and yes you can change the Gamerzone after you choose it the Gamerzone is basically like choosing you preference of play like just to have fun or your very competitve so you would go with pro because your competitve.