There may be no actual minimum age, but there is an implied minimum over 18 policy in effect that most retailers I know of use, granted my knowledge of retailers isn't huge but I do know a lot of the distributors in the UK and they tend to check this is the case.
I'm not sure if anyone has shared this
http://www.sciencedirect.com/science/article/pii/S0273230014002505 bit of research but it makes for interesting reading I find.