At What Age Is a Mastiff Full Grown?

He doesn't reach his full size until approximately age 3.
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When fully grown, the mastiff is one big dog. The American Kennel Club standard for the male mastiff specifies an adult height of at least 30 inches tall at the shoulder, with females slightly smaller at 27.5 inches and up. Expect your mastiff to tip the scales at 200 pounds. However, it takes this giant breed longer to mature than other canines. They aren't considered fully grown until the age of 3.

Mastiff Growth

It's not just physical maturity that takes three years for completion. The same is true of the mastiff's mental state. He's in "puppy mode" far longer than other dogs. Like other large breeds, mastiffs are especially prone to bone and joint problems, so avoid strenuous exercise with dogs who haven't reached maturity, even though they appear big and robust. Ask your veterinarian about the best diet for your growing dog, since you don't want to feed him excessive protein. That can lead to more rapid growth, which isn't healthy for your mastiff.