It takes some level of maturity to be on a disciplined team. Some young players have better discipline than adults, so it is always good to have exception clauses for talanted, respectful players, regardless of age. Eighteen is a good age limit for a team for several reasons. Fourteen year olds will get bored with a game quicker, and move on to the next new game, making it hard to keep the ranks filled. At eighteen and older, they tend to stick to the game longer, and the team can last longer. Sometimes a young team leader lets his ego get too big, and starts making demands of players, making it harder to keep the team functioning. If you're under a team's age limit, try to impress them with your maturity and respectfullness, don't start telling them how they suck because you can't get on their team. That approach rarely works. A personal story: The team I am with has an 18 year old age limit, and sometimes we make exceptions for 17 year olds and let them on. If a player has a son or daughter, we will let them on regardless of age, so there are some 12 year olds on the team, but they have their parent's approval and guidance. Many times a player will ask to join, and be under age. We ask them their age, and they tell us 15 or whatever it is, knowing that they are too young. Our standard reply is that if they lied to us, they probably wouldn't last through the screening process, and that anyone who wants to be on the team, will be telling us the truth from the start. We will be around in 2-3 years when they are old enough to join, and they know that.
before you join a team, ask how old the team leader is, and what the average age of the players are. You will have more fun playing with people your own age than with a group much older or younger than you are. Youinger teams will be more relaxed on the rules, while older teams will usually have more rules, and enforce them more.