No, you can start whenever you want, but i wouldnt recomend working there
Yes, they need to be 18
You basically just help sell stuff, you need to have product knowledge. Often times you will have to meet sales goals and sell a certain dollar amount per day. The age depends, I have most recently heard that you do have to be 18 to work here.
You can be a sales associate at any age. 18 and younger may not have the requisite products knowledge which would be a disadvantage