There is no single set age when a dog becomes a senior.
Size can help as a guide:
Signs of aging can include a greying muzzle and slower walks.
Senior pets are more likely to develop health issues. Catching problems early matters.
Senior pets should see a veterinarian at least twice a year.
A pet being treated for a health issue may need more frequent checkups.
At visits, the vet pays attention to:
Older pets slow down and become less active. Age-appropriate exercise still matters.
Exercise can help minimize loss of muscle and strength. It can decrease weight gain and obesity. It can also help with joint pain and stiffness.
How much exercise depends on how active the pet has been recently. It also depends on what health issues it has.
If a pet has not been very active, start slowly with short walks. Increase them as the pet gets used to them.
Consistency is the most important thing.
Small changes at home can help a dog get around.
If the dog likes being on a bed or couch, consider adding stairs. Pet steps can help it get up more easily.
Moving the dog's bed closer to where you sleep can make it more comfortable.
Moving food and water bowls and the bed to the main floor can help. Then the dog does not have to use stairs.
A ramp to get outside instead of stairs can also help.
As pets age and slow down, their metabolism slows too.
Weight gain can be hard on a senior pet.
Consider changing to a diet made for senior pets. Consider giving lower-calorie treats.
Older pets may have trouble keeping themselves clean. They may have trouble reaching areas to groom.
More frequent bathing or trimmings may be necessary.
The coat may need more frequent brushing. This keeps it from tangling. Tangling can cause discomfort or difficulty moving.
Check the nails regularly. Nails that get too long can make it harder for the dog to get around. This is particularly true on hard surfaces.
ASPCA, "5 Expert Tips for Caring for Senior Pets," aspca.org — tips offered by ASPCA Poison Control experts.