I'm not admin but can probably answer your query.
The age requirements at Stirling events is quite strict as they do tend to hold their events on MOD sites and in remote locations for extended periods (24+ hr games). As such, the limitation on the age of attendees at these events is in place for legal, insurance and safety reasons.
I think your enthusiasm to get involved should be applauded but at the moment, you would be too young for a full-on events like these. I would reccomend that you get a taste for actual airsoft at a skirmish site local to you, some of which likely will cater for younger players.
Don't let it put you off though - I'm sure Stirling will still be holding events of one form or another by the time you are old enough to attend!
You might want to look on the ASCUK forums for contacts and skirmish sites in your local vicinity:http://www.airsoftcommunity.co.uk/forums