When it comes to dietary needs, every dog is individual - regardless of age. So there's no one easy answer to the question of soft versus hard food. Both dry and soft food can provide your dog with the nutrition your dog needs; consult with your own vet for more information about what food best matches your dog's needs.
That said, there are a few reasons you can choose to feed him soft food.
As your dog gets older, his teeth may become more sensitive, which can make chewing kibble more difficult and even uncomfortable for him. Switching to a soft food will help to alleviate his oral discomfort when eating. If your dog is experiencing serious pain at mealtime from a condition like tooth decay or gingivitis, switching to soft food certainly won't remedy the problem. Make sure to talk with your vet concerning dental treatment.
Again, digestion begins in the mouth, so if your dog has a tendency to bolt down his food, he may not be adequately chewing or adding saliva to it. Soft food can aid with digestion because it's more easily broken down and therefore more easily digested.
Whether you choose dry, soft or a mix of both foods, ask your vet before taking any action.