My current vet told me the same thing. Yoshi was a puppy mill rescue & I was told by the Lab breeders that I adopted him from that he was 20 wks old. During my first vet visit, my vet told me that he was probably 4 months old because he had no adult teeth yet. His grown up teeth started coming in a few days after our vet visit.
I'm no Yorkie expert...YT has been a godsend for me! But, I'd like to believe that not all Yorkies develop adult teeth at the exact same time & that perhaps my Yoshi was just a late bloomer.
I was really concerned at first because a month is a big difference when you have a new pup & you're trying to be the "perfect mommy". But eventually, I just decided to compromise & basically told myself that he was 4 1/2 months when I adopted him. Now that he's "7 months old", I'm okay with it all.
I suppose unless you go through a reputable breeder, you never really know...
Good luck! Your baby is a cutie patootie!