Is there a legal age?

My son is into gunge and custard pies etc. He's always finding something new to be gunged with but has expressed a desire in finding someone else who is willing to partake. Now, he's only sixteen so i'm worried that if he had a partner to do this with that it could turn sexual. Obviously, the legal age for sex is 16 here in Britain but is there a rule for this sort of thing or is it just 16? It would put my mind at ease if I knew as I know he's sensible but I wouldn't want the age to be something higher if so as it would get me more apprehensive.