How can i know his age? ?

I adopted a male boxer about 3 months ago and at the shelter they weren't sure of his age. The owner relinquished him so I got him before time and I guess the owner said he was 7 years old and an escape artist (which obviously isn't true) and he was VERY neglected (his teeth had been grounded down and all his bones were visible) but the vet told me he didn't look 7 years old that he was a young dog. I would like to know if there's a way to know my doggie's age