Following on from the wise Bunza...
There are three types of licence: The first allows the sale of alcohol to anyone from 11 am to 11 pm, generally without conditions The second allows the sale of alcohol to anyone from 11 am to 2 am. (to get one of these the venue must already have a Public Entertainment Licence) The third allows alcohol to be served if taken with a meal. So while a full licence (the first type) would cover drinkers from 18+ and therefore those aged 21, it will depend on what special conditions are being sought to go with it.