How long will my cat live?

Jasper is a 3 year old out door cat.He is exercised a lot ,and is fed daily. He is slow now, and doesn't play or pounce on my other cat Wallie anymore. But occasionally he will play ruff. I was watching Cats 101 and it said that out door cats live 2-3 years.... Is that because they could be more exposed to diseases? He is a barn cat ,but is a normal sized cat , would that affect anything? Any awnsers are needed!!!