for some
reason I always thought dogs live to around 20 years, probably from
something my parents told me when I was young.

20 years is *highly* unusual. Average lifespan of dogs is, IIRC, 14- with
larger dogs generally living shorter lives (giant breeds' average age is
something like 9) and small generally dogs living longer.
So yes, 12 1/2 is getting up there for a Lab... one of our semi-regular
posters often uses a sig line which says "Dog's lives are too short- their
only fault, really."