As Loonytunes indicates, the Licensing Act 2003 removed the former rule requiring staff serving alcohol (in pubs) to be 18 years old. The situation is now the same as you see in supermarkets, where check-out staff (who're under 18) have to get authorisation from an adult colleague for each and every sale.
In a supermarket, the manager can't give 'blanket' permission for young staff to serve alcohol. Every single sale must be separately authorised. The same position now applies in pubs. Staff who're under 18 years old may serve alcohol but only if each sale is separately authorised by an adult. In practice that means that most pubs would never consider employing someone (behind the bar) who is under 18 years old.
As Loonytunes also indicates, it's illegal for anyone to employ any person under 13 years old. At 13 years, there are only certain specific jobs which young people can undertake. (These are listed by the relevant local education authorities). These would not include any job in a pub. From 14 years onwards, until school leaving age, a young person can work in any job for which the local education authority is prepared to issue a permit. A permit might be granted for work in a pub but probably not if the work involved serving alcohol. (Anyway, a person who has not yet left school may not be employed after 7.00pm, so it's unlikely that they'd be offered a job in a pub anyway).
Once a young person has reached their school-leaving date, they can work in a pub but they can't serve alcohol unless each sale is separately authorised by an adult.
Chris