My oldest piggy is currently 8 years old, I have another of 7 1/2 and quite a few of 5-6 years. Plus some young ones!
I'd say 5-6 is a good age for a guinea pig. I've had a few die at 4 years or so and have usually regarded that as normal too unless there was something evidently wrong with the piggy. I think some of it comes down to genetics and some of it environmental. Plus piggies are good at masking illness and sometimes they only seem ill when it's really too late.
I've had 1 or 2 mysteriously die at less than a year. I had a gorgeous DEW called Magic and she was probably 10-11 months when I just found she had passed away. She'd been fine a few hours before that, that was very puzzling.
Someone I know has the theory that the guinea pigs that drink the most and need their bottle topping up more regularly live the longest. I'm not sure how true and correct that theory is though, I don't have a lot to base it on!