What age do you need to be to work at the following companys?

Hi, i'm currently looking for a part-time job in my local town. I was wondering if there was any age restrictions on how old you have to be to work at a mobile networks shop, as we have an EE store, a vodaphone, a o2 shop, and a three store.

I was also wondering if they have any internal age requirements for the job, as i'm only 16.



Many Thanks!
Jacob