Bullmastiffs

I adopted a BullMastiff from the pound. He is the sweetest most loving dog we could have hoped for! The vet that nuetered him suggested he is about a year old, but we noticed the other day he had lost a tooth and there was a new one coming in. He is about 100 lbs already. I wonder if a year old is about accurate? Although ultimiately it doesnt matter, we love him no matter what. Its just my experience in the past that it seems alot of vets say somewhere between 1-3 years on dogs that are adopted. I appreciate any insight anyone has! Thanks!