There is no minimum age in the Licensing Act 2003. This states that under 18s can serve alcohol on licensed premises under the supervision of (i.e. each sale/supply specifically approved by) a "responsible person". This approval is not required when serving alcohol in conjunction with table meals.
However, children below the minimum school leaving age are also subject to legislation in regard of employment (such as a minimum age of 13) as well as local authority bylaws which restrict types and hours of work.
This is why, I imagine, whenever I buy alcohol in Tesco, the younger checkout assistants have to get a supervisor or an older assistant to 'watch' the sale.