i would not feed a healthy dog senior food at all.
the reasoning behind this is that most senior foods are just drastically reduced in protein and fat, which are the two most important things a dog needs in the diet. older dogs with a slower metabolism have to work harder at extracting nutrients, so decreasing important ones is not something that supports their health. if the dog is rather inactive, reducing the overall amount fed daily by a little bit is the better way to go.
that being said, there are a few senior foods on the market now that really are improved for the specific needs of older dogs and not just reduced in calories. these foods have a higher protein content and a reduced amount of fat (but with an increased amount of essential fatty acids) and are often formulated with lower levels of phosphorus to take stress off the kidneys. one of these is eagle pack senior care.
as for when is a dog considered a senior?
that depends on a number of things, breed and genetics first and foremost. a large or giant breed like a great dane or mastiff could be considered a senior at age 6 since they don't have a very long lifespan to begin with, 10 years being on the high end of the scale already.
small dogs often have a life expectancy of 15+ years, so 6 years isn't even a "halfway mark" for them yet.
i do not agree with the arbitrary labeling of dogs as seniors which is mainly perpetuated by the pet food industry giants like hill's, purina etc. their only goal is to sell you yet another "specialized" product and in the case of senior food (reduced protein/fat, increased carbs) that means a greater profit margin for them.
my own great dane lived to be over 11 years old and ate regular adult food all his life. my wirehaired dachshund will be 13 1/2 in march and also does not eat senior food, in fact i was able to convince my mom (whom she lives with now) to switch her to a mostly raw diet with lots of meaty bones.