Good morning,

1) Yes
2) Yes, depending on the age
3) Dependent on the dog's general health and activity.
4) Yes.

When my late Bo, began showing signs of his senior age: his regular food starting disagreeing with him, and his activity level dropped. Our vet did his physical to include blood work. This was when we changed his diet to his senior. He was about 8.5 -9.

Up until then, his symptoms weren't noticeable. As for changing sooner, the packages are guidelines. Changing to senior food is best done with guidance from your vet and your ability to notice the changes in your pet.

The nutritional changes between foods are designed to meet the changes in dogs. They may have the same basic ingredients, however, the vitamins and oils may increase/decrease depending on the type of food.

The other big difference in foods are the brands. If you are using the commercial store brands, (ex: Purina, pedigree, etc) you are getting a lesser nutritional quality food than those of your premium brands (iams, eukanuba etc). This too also makes a difference.

Remember each animal reacts differently to changes. What worked for my Senior, may not necessarily be the same for yours. Only your pet can really tell you by his actions and health.

Good luck.