18, else the question would be pointless. if the adult price was charged from 16 say they could charge you the adult price but say that you look 16 or 17 to them.
Yes; the cinema I worked at charged adult price for anyone going into an over 15 film but that doesn't make it legal for them to sell alochol at the bar in the 'Deluxe Scree'.
when you turn 16 you pay full adult fare and get none of the adult problems or privileges. When you turn 18 the onus is on you to prove you are 18 even if they display a 21 years old or challenge 25 sign.
Ah I see, I suppose you could challenge being charged adult entry but don't know whether you'd get anywhere because; as you say it's challenge 25 for alcohol.
Unless things have changed you need to be 18 to be served alcohol so anyone under 18 would be refused..........that is the law. Organisations can provide services at reduced prices for children with the adult rate kicking in at quite arbitrary ages.....that's up to them.