I've worked for the Co-op for 14 months. I'm 19 and I earn £5.85ph - everyone earns the same over 18.
As for what it's like - it varies from day to day. Some days will be good and some you'll just wish you'd never been born.
As with every sales job, if there's something not in stock, or you've not got any £5 notes etc. then it's automatically your fault, even though all you're paid to do is get stuff from the warehouse to the shop floor and sell them at the till.
The worst part is age-restricted sales. With the new under-25 policy, we've had to ID people we've not in the past, and they can get angry!
But most of the time it is alright - and in fairness it is a job, it pays and to have one at the moment is no bad thing.