what's the average life span of a dog? and btw, anyone knows what are the symptoms before a dog die?

depends on the breed

he is a mongrel

bad vision, bad legs, difficulty walking/running.

Or bad appetite etc.. when a pet stops/nearly stops eating, it's always a sign something is seriously wrong.

um wat if eat 2 much

Isn't it around 20 years?

It depends on the breed and average weight of the dog. Small breeds tend to live longer than large breeds, while mixed breeds may have fewer problems (or more, if you're really unlucky).

Pretty everything that was posted above is a good clue. If the appetite suddenly stops, or you notice him having a lot of difficulty walking or seeing where he's going, it may be time to see a vet. However, please don't be cruel and just let the dog wander around until he passes at home if it looks like he's suffering at all.

13-14 years is a VERY long life for a dog

so 20 is pretty rare

Yeah, 20 is a serious feat.