@Droite
Just because they want it doesn't mean they need it.
There's so much we want but don't need. Alcohol, cake, cream, cosmetics, to name only a few examples.. Should we be paternalist about one section of society but not others?
The section of society being children, though. Towards whom we are consistently "paternalist".

We're supposed to be. We're mammals, not turtles.
May I remind you what happens if a minimum-wage member of staff sells alcohol to under-eighteens? A fine, that's what.
As of 2012, the maximum fine for repeated offences (defined as two or more different occasions in a period of three consecutive months) of selling alcohol on the same premise to a person aged under 18 is £20,000.
If Keira had managed to buy WKD from a pub twice in the run-up to her 18th birthday, what, realistically, would the consequences for Keira's health be today? (And yes, I do agree with the Chief Medical Officer that children should have an "alcohol-free childhood".) To go by the average mumsnetter's tales of teenage fun, there would be no consequences. But £20,000 fine nevertheless.
Meanwhile, in America, you can't drink alcohol until 21, if I've got that right.
Surely what has happened to Keira (cross-sex hormones and an elective double mastectomy) is far more serious and has far greater consequences for her future health than being sold 2 WKDs by a 19 year old bartender?
Surely the responsibilities and consequences for graduate and post-graduate NHS staff should be greater than they are for a 19 year old bartender who sells a 17 year old alcopops?