Any age you like, as long as they are supervised/have permission of some one over 18. (I think). I know the law has been changed so that under 18s can serve alcohol but can't remember the details. (Too much to drink)
When I did barwork 10 years ago it was you had to be 18 to be emplyed to serve alcohol, a younger person could be employed to collect/wash glasses.
Of course a lot has changed in the past 10 years so this may have too. I do know, however, that when buying alcohol in a shop/supermarket, the person serving has to call their supervisor if they are under the age of 18.
I have recently taken the new licencing course (in the UK) and the new law is that every sale of alcohol has to be approved by a Personal Licence Holder. the Personal Licence Holder can approve under 18s to serve. The way we have got around this new law is to have a list behind the bar with all the names of the staff on (under 18 or otherwise), their signature saying that they understand the new law and not serving under 18s/drunk people and the personal licence holder has also signed to say that they approve this person to serve alcohol on behalf of the personal licence holder. sorry if this sounds confusing, it is!