No idea on the diagnosis but I'm surprised this was the first time a vet suggested that your young dog might be a senior.
The vets around here start recommending senior panels at the age of seven. I agree with seven for larger dogs, and I agreed for Sunnie because I really had no idea just how old she was, but I'm going to stall on 35 lb. Dan for another year (he'll be eight the last day of this month). Senior checkups are much more costly than regular wellness checks and I have no reason based on Dan's history to suspect that he needs more. Urine and blood are already part of the regular exam.
Good luck with Sierra.