When my cat was diagnosed with cancer at 11yo, I asked my vet the same question. She told me a cat can live healthy till 20 if kept indoors, an outdoor cat about 2. It told me ALOT about whether or not keeping my cat indoors was the correct thing to do!
My cat made it to 17 with the cancer. My sister has a 19yo cat now who has arthritis but is spry like yours!
Lifespan ALWAYS depends, the average lifespan for a human is 76.9 and the Today show contests that everyday!