When they start to get old you can give them very low doses of seleginline "called anipryl" for dogs. It can be bought online or just get it from your vet.
Do some reading on it, if started earlier it keeps the brain younger and your dog won't experience any of the age related brain deterioration effects that will inevitably come with old age. Works on humans as well, it was actually designed first for human use. It keeps the brain from losing it's ability to produce the normal amounts of dopamine and norepinephrine.
They mostly prescribe it after your dogs mental state starts to decline but I would think the earlier you start it the more benefit you'd see in the long run.