How many years do cats normally live for?

18 years on average. I had one cat who lived to be 18, and one who lived to be 21. The one who died earlier was the one I expected to live longer, and I never expected the other cat to live for 21 years. Both of them were put down, the younger one had to be put down because a lump was growing in her mouth that was stopping her from eating, and the other one I could notice getting thinner each day, the vet couldn't even feel her kidneys, and she wasn't really eating. She had to be put down in November in 2008