Well my Dad would argue about that, he's not 90 yet but he's in pretty good shape at 86, he doesn't need assistance to get around or do anything, has better eyesight than I do, only thing wrong is his hearing and that has as much to do with his military service as age (he gets a Navy pension because of it)
Genes have a lot to do with it I reckon, I've been doing my family tree and particularly on my maternal side there are folks (mostly female) who have lived well beyond the average for their time, my grandmother reached 100, her mother, who was born in 1890 reached 93 and I've found a number of others in the 1800's that reached 90-100, surprisingly for the times the infant mortality rate is well below what you'd expect and given the number of kids they had in those days I have a lot of relatives!!
Back to dogs, our mini schnauzer reached 19 until she was put down earlier this year, she was a rescue from a puppy farm and was in terrible shape when we got her - my biggest regret is I didn't get 5 mins with her original owner