15 years is very long for these dogs; that’s more a small dog’s age. 12 at best; 11 probably average.
As for DM, here’s a little group on it: http://www.mzjf.com/
My accountant had a Golden that lived to 16. Of course, I’m spoiled because my previous Golden made it to 15 1/2, and she had a good life until the last 1/2 year when she really began to degenerate.
But, I’ve known several people (including my son) who lost Rottweilers at between 8 and 10 years. My daughter’s Standard Poodle, on the other hand, made it to 16 1/2.
I always figure, however, that anything beyond 10 years is a bonus for a large dog. Unless it is MY dog, of course. I want MY dog to live to 20! LOL