It depends on two things: The site insurance and the wishes of particular game organisers. Some sites insure players as young as 12, some 16, some 18. Some want you to have an adult with you, some don't.
Some organisers only want over 18s at an event, regardless of the site rules.
If the age limit isn't made clear when a game is announced, feel free to ask 
All the game run this year by Gunman Airsoft - so that includes the one last weekend, 18/19 August, 17/18 March plus two other organiser games games on 4th Feb and 26th May
http://www.gunmanairsoft.co.uk/
Email to: enquiries@gunmanairsoft.co.uk
Look out for 2013 games to be announced on this forum - there will be games in March, May, July and November that under 18s can attend (and could be more)
how old are you mate? the fuhrers escape is in 3 weeks and over 16 is fine, but you must have a responsible adult with you ?
"Take that you rotton helping of strawberry flan!"
Joseph Porta to "strawberrys and cream", in the sven hassel book ,ogpu prison
