Look at the back panel. They give the nutritional analysis there. Younger dogs want more fat and carbos, midlife wants less of these, and a good mix of proteins as a source of amino acids. Late life wants less protein and more carbs.
Every breed differs somewhat in its nutritional needs as well. Big bones in a St Bernard want a different growth diet than a pug. Pretty good breakdown by breed at
http://home.att.net/~cusickbook/2.pdf
Level of activity determines how much you feed. As does your climate, and whether the dog is provided with heat or has to make his own.
Our German Shepherd is nine, and his schedule is now two smaller feedings, where it was once one large per day. He is supplemented with corn oil for the omega-3 and a good coat. Purina chows have taken him from puppyhood through this stage, but he's definitely a "senior" now.
All that being said, any good commercial manufacturer will provide analysis for age, you provide the portions and supplements. We feed dry, and provide plenty of water with NO Chlorine or Flouride for him to drink. Dry food provides good mechanical action to keep the teeth firm and clean.
When we had dogs with narrow mouths like our Collies and Borzoi, we softened the food so it wouldn't pack against their teeth. Had to brush them once in a while too, though bones or rawhide helped minimize the need to gag them and us.