According to veterinarian Akiko Fukushima of the Japan Animal Medical Center in Tokyo's Shibuya Ward, while there is variation by breed, age and other factors, dogs grow rapidly in their first few years and afterward, each year is a human equivalent of around four to seven years of aging. In general, dogs are considered aged from around 10 to 12 years old for small breeds, 8 to 10 years old for medium-sized breeds and 6 to 8 years old for large breeds. Like humans, when dogs age they undergo changes in appearance, such as their weight or their coat color becoming pale. In some cases, they can require care due to mobility loss or inability to use the litterbox. It's also said that the animals experience a decline in interests and enthusiasm as well as changes in lifestyle rhythm due to declining cognitive abilities.