Senior dogs have different care requirements than those of a younger dog. This fact probably doesn’t come as a surprise to anyone.
Giant breed dogs age faster than smaller breed dogs. A Great Dane is considered to be senior by roughly 5-6 years old whereas a Chihuahua would likely only be middle-aged then, and probably not considered a senior until 10-11 years. Large breed dogs fall somewhere in between. A Golden Retriever might be considered senior by 8-10 years of age. Genetics, nutrition, environment; all of these play a role in how fast your dog ages.