Size doesn't matter.

How often was it fed before you got it? How old was it when it got it's first meal? What size feeders has it had before and after you got it? Has it endured any abnormal stress before or after you got it? Has it been excersized regularly before and after you got it?

Cannot really guess acurately the age without a detailed history of it's care. Of course if you had the kind of detail needed, you could just look at the first date on the record and know the age.