Wow. What a minefield of a topic. As with human nutrition you will find so much information out there. What to feed, when to feed, how much? Then there is the added confusion of the different nutritional requirements of your pet throughout their age.
Lets break it down and try to simplify things a little. So age groups:
Growing puppies should be fed a nutritionally balance diet of a good quality biscuit. There are some good quality wet food options available as well. Initially when you bring your new pup home given 3 meals a day is a good idea, until they are about 12 weeks of age. After this you can reduce to 2 meals a day and in some cases after 6 months of age some dogs can be reduced to 1 meal a day.
The type of food depends on your dogs breed or expected adult size. There are some great options ranging from small, medium and large breed options. This is because different sized dogs will require different amounts of energy. Just think how much a chihuahua grows over it first few months of life compared with a Great Dane.
If you have a dog between 12 months and 7 years, with no health concerns they can be on a good quality adult biscuit. Many of these diets also include some dental health benefits which is very important in maintaining dental health.
A dog is considered to be in general older once they reach 7 years of age. However we do know that larger breed dogs do tend to age fast than the smaller dogs. Again there are some diets formulated to help target the needs of older dogs. For a healthy dog there are generalised senior diets that assist with the lower caloric requirements of your old and possibly slower dog.
Any dog with health concerns
If your dog has any health problems, then your veterinarian can help make a nutritional plan for your dog. There are great diets formulated to help manage diabetes, kidney disease, sensitive stomach, sensitive skin and arthritis just to name a few. These are called prescription diets as they are specially for particular health concerns. These need to be prescribed as part of your pets all round health regime.