Teeth are about the only way I know of that someone can guess an approximate age. Have you had Sophie spayed? If not, you might be able to tell by that also because all my Beagles seem to go into heat at about 9 months of age for the first time. Other than that...you will probably never know.
If your breeder is like me though they can tell you exactly what dates and times their dogs tied and the exact time of the birth of your individual pup. I am very anal about my record keeping. lol
You can't tell by weight on a beagle that is for sure. Some end up being such petite little things and others more like a moose. Maya is a moose, Tate is short but solid and Sulli is more of a petite little lady. Both Tate and Sulli are 1 1/2 years of age and one weighs 25 lbs, one weighs 32 lbs. You just can't ever tell.
If the vet says the breeder was accurate...I'd believe the vet.
I don't think weight is a good indicator. Cole is only 6 months old and weighs a whopping 22pounds where Roscoe is a year and 8 months and weighs 22pounds. I don't understand it because we feed Cole the exact same way we fed Roscoe at that age and he is just HUGE! We have him on adult food now b/c of this!
It's hard to gauge by size. I am friends with one of the families who adopted Jersey's sister. Jersey is about 16-17 inches tall and weighs about 40 lbs. She's a big Beagle girl. Gidget, her sister, is about 3-4 inches shorter than Jersey and only weighs about 30 lbs.
If your vet says the same thing the breeder told you, then it's most likely correct. You could always try a different vet to see what they say, but why waste the money? I''ll see if I can find the paperwork from when Jersey was a puppy to see what her weight was (I save all that stuff...I still have Amber's stuff and she passed to the Bridge almost 4 years ago).