It depends on the breed of dog, as well as its health. Smaller breeds mature faster, but age more slowly after reaching maturity. A small dog (such as a toy poodle, terrier, chihuahua, etc.) isn't considered "geriatric" until much later than a larger breed - maybe 10 to 12 years old, and the largest breeds are senior citizens at 5 or 6 years of age. Most vets, though, consider a dog 7 or 8 years or older to be a senior citizen.
Some 'senior' dog foods are basically marketing hype, with little difference between the regular food and the senior food. All young dogs should be fed puppy food as they have very different requirements to adult dogs, but whether or not an older dog is switched to senior food seems to be a matter of the owner's choice as often as the dog's health.
The switch to senior food should be considered carefully and your vet should be involved with this decision. Some dogs become less active as they age and therefore need fewer calories and nutrients. Other dogs need more due to poor digestion. Senior foods that can be bought in supermarkets rarely describe exactly what is in them compared to that brand's regular adult dog food.
If your older dog has specific health problems (such as failing kidneys, Diabetes or poor absorption) it will need a different blend of nutrients to healthy, younger adult dogs that may or may not coincide with senior foods. Senior foods may help older dogs to live happier and healthier lives, but they may also be unneccesary wastes of money. Also, the foods that contain less energy and nutrition may be detrimental to the health of dogs with poor digestion.
The age at which each dog should change food will depend very much on the individual situation.
An antoynum for senior is old
The best bet when you're trying to find good brands for senior dogs is to go with a well trusted brand like Iams or Purina. They have products specifically made for senior dogs, but you might also consider talking to your vet.
adult dog food is for adults and senior dog food is for senior dogs (older)
You must be at least 50 years old to play in the Senior PGA.
Most dogs are considered senior when they reach around 7-10 years of age, depending on their breed and size. However, larger breeds tend to enter their senior years earlier, around 5-6 years of age.
The best brands are Iams and Eukanuba
Elderly or senior.
To qualify for the PGA Senior Tour, golfers must be at least 50 years old.
To qualify for the Marriott senior discount, you typically need to be at least 62 years old.
You must be at least 60 years old to get into a senior community
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Typically, a student is around 17-18 years old when they are a senior in high school.