What is the minimum age to be bar staff?

You need to be over 18 to sell alcohol.  This is whether you are in a corner shop, a supermarket, a pub or a bar.  Just as if you are working in a supermarket you will need to ask for an older staff member to approve the sale of alcohol, you would need a similar system in a bar.  So you do not have to be over 18 to work in a bar, just to sell the alcohol.  For example a pub could employ you to be a waiter or work behind the scenes until you turn 18, at which point you could legally sell alcohol from behind the bar.

The employer would also need to consider other aspects of your working environment for example working hours and health and safety, particularly while you are under 18 years old, to ensure they are conducting their business within the law.