First 5 is probably borderline at best for calling a Rottie a senior dog. In general the larger the dog, the shorter the life span.

The general rule I've used on food is if the dog is at a reasonable weight, and its energy level is high, then the food being given is optimal.

For my previous dog I fed her a performance formula until about 5. At that point if I gave her enough to satisfy her hunger she would gain weight. So I switched her to a maintanance diet - less calories per serving size, therefore more could be given. This worked out well for the rest of her life.