Rates of aging vary by breed and size. Smaller dogs generally have longer lifespans than larger breeds, although each individual dog ages differently. Usually, the larger the dog, the sooner it begins aging. Some giant breeds can show signs of aging as early as 5 to 6 years, and some toy breeds show no signs of aging until they are 11.
The folk tale that dogs age seven years for every calendar year is inaccurate. It's more accurate to think of dogs as being equivalent to teenagers by their first birthday, in their mid-20s by age 2, and then aging about four human years per calendar year after that. So a 3-year-old dog is about like a 28-year-old person, and a 12-year-old dog is comparable to a 64-year-old.
Once you know how "old" your dog is, you can devise a senior health plan with your vet to minimize the impositions of aging and prolong the time you and your dog have together.