UCAP have a policy and let me see if I can remember the wording correctly from their morning safety brief,
"We do not allow the use of two tone or clear guns or the use of guns bought outside of the VCRA system at this site" (OWTTE) They also don't allow them to be covered for the days play if I remember correctly.
I've definitely seen plenty of younger players down at the sandpit, 14 perhaps and they all played outstandingly well, they encourage it. I think its mostly the realism issue, it really spoils my day when I see something highlighter orange sneaking through the woods especially when the carrier of said object thinks they are the Predator in stealth mode. Especially when the gun is used as a ref point to call them out to others and a rain of fire pours in on them.
Two tones are not for under 18s only, remember you should be over 18 to purchase any airsoft gun, you just need a defence for a RIF.