What is the age limit for pet stores to display guinea pigs?

I got my guinea pig today from Petco and was very dissatisfied by the workers. First off I had to go & get a guy who worked there because there weren't any around. They were all hanging out by the cash-register. Anyway, when I pointed to the one I wanted I asked how old it was and he didn't know. I was just wondering how old guinea pigs usually are when they are up for sale?