Is there a way to estimate age of guinea pigs?

I recently purchased two guinea pigs, 1 came from petsmart and all they said was that it was a baby. He's approx 5 inches long and kinda on the bony side. I went and got a second from Petco because the other store only had the one and I wanted two. So, anyway with the second I was told he was a baby and only 6 months old but he is almost twice the size of the other. So, I'm curious if petco lied as to how old the larger guinea pig is or maybe the smaller one is sickly or something.