what age can male dogs stop mating at?

When no-one wants to use him!

Seriously, though. So long as the stud dog is fertile, and still passes any yearly test (eye test, for example) and someone wants to use him and the owner agrees there is nothing to stop him being used all his life. There is no 'use by' date on siring litters. Only on bitches.

He would need to be fit and healthy, though. It's tiring work! Especially for the owners :D

It depends in what context you ask. Physically, a dog is capable of mating up until the day he dies of old age. Stud dogs are usually retired by about 11 or 12 for the good of their own health, and the fact that their fertility may well be waning. But it doesn't stop them wanting to try!

If you're asking if it is safe to leave an elderly dog with an in-season bitch, the answer is a resounding NO!

When I was Health Visiting, I visited a family who had sent their bitch to kennels during her season, as they had a very elderly male dog at home. When she had finished (?) she came home, elderly dog gained a new lease of life and sprang into action - one singleton puppy resulted! :D :D :D
Cathy.

I used a male who was 11, don't know about him having a heart attack but I nearly did worrying about him, never again :d