Where alcohol is served with food (e.g. by a waiter/waitress), there is no minimum age.
When alcohol is served on its own (e.g. at the bar) any person under 18 must have permission (for that particular sale) from a licensee (or any other adult designated, as being allowed to give permission, by the licensee). The situation is similar to what happens in supermarkets when a check-out assistant, under 18 years old, sells alcohol. He/she can do so, but must seek permission from an adult colleague. (The store manager can't give 'blanket permission' to the young person. Every sale has to be individually approved).
Chris