Moomintroll's answer is 100% correct but, just in case it's not clear, I'll add that 'supervision' requires each individual sale to be authorised (in the same way that young check-out staff have to ask permission every single time they sell alcohol). The licensee can't simply give 'blanket' permission for a minor to sell alcohol.
Since very few publicans would be prepared to authorise every single sale on a busy Saturday night, the minimum age for working behind a bar is effectively 18, even though the law doesn't actually define a minimum age.
Chris