To take a position as Devil's advocate - anyone under 18 is a child. Children should have their own game time (and date) separate from adults only skirmishes. Let there be family time where children can play with parents, older siblings, and friends. Maturity is subjective, stereotypically every child thinks that they are more mature than they really are. Whilst men will remain immature forever, you need to draw a line somewhere and 18 is the point where you can vote and drink. Based on apprentice soldiers I would say 16 is the youngest age for an adult skirmish. Children are more prone to being sensitive and not as emotionally tough, so they cannot deal with piss taking or listen to advice, and may well misread the situation. Adults just want to play airsoft without dealing with tears and tantrums.
Yes, we as a hobby need to encourage new members and instil in them a sense of sportsmanship. Adults can be arseholes but airsoft is a cross section of society and nothing to stop chavs and cheaters from playing. I do not condone treating children, or any other player badly. However, there is also the fact that children themselves can be rude and lack respect for anyone else regardless of age. To date I have never seen any problems with children attending a skirmish, although have not seen many. Recently had a father and young son playing together and no issues, for me the worse aspect was feeling guilty when I shot the boy at close range (whereas shooting an adult would not bother me, but that could just be conditioning not to hit women and children).
At the end of the day, play to your best, take your hits, and have fun. Then post on the forum all your funny moments, gripes, and any other random crap that is in your head.