There is a Booths supermarket spaced. And i think it's 16, but they won't be allowed to put you on a till, as you need to be 18 or over to serve alcohol.
The nearest Booths to me is in Lytham and is always full of really posh people with loads of money,I hate going in there.lol.You get much less snobby people in Marks and Spencer and the food tastes nicer :-)
I know our local Morrisons won't employ under 16's so I would think most supermarket chains are the same.