I am far from an expert but I disagree with your vet that it is an old age thing.
As a human, if you suddenly had the same problem and it was never an issue before, would you just think it was your age? Something isn't agreeing with his system.
If it was me, I would put him on plain white rice and boneless, skinless chicken for a week or two and see if it clears up. If it does, there is something in the brands of food you are trying that are causing the problem (corn, wheat, etc). If it doesn't clear up, I would put him on just chicken for a week or two and again see if it clears up.
If you don't want to go that route, I would by one of the true top of the line dog foods with minimal ingredients on the label.
I have owned and cared for many dogs (including several labs) and this was never something that happened when they got older. Good luck.