...marshall at any age... but there may be some problem with insurance at the site... some sites don't let under 12s play so I can't see people that young marshalling either!
I think it will probably come down to an insurance thing.
If the nearest site is miles away it won't be worth it!
If you have a site that you regularly play at just ask the owner/manager dude, thats how I did it!
You can only ask, what have you got to lose?