I would say my oldest is around 2 years old but that's because I only got into bettas around the same time. He looks a bit haggard as he had a slightly turbulent start (my cycle crashed in a big way and it was a week or so before I noticed).
But I say clean water, nutritious food, a stimulating environment with space to self-exercise and a couple of flaring sessions a day is a good start to getting your betta into the twilight years.
A fit fish is a healthy fish, which is why it's so important that your tank is warm enough and big enough to encourage your betta to spend most of its time moving about. There is the real danger of fatty liver disease if you are feeding a rich diet to a betta in a very small space over an extended period of time.
I've have admittedly found that all my oldest bettas have lived in cycled, established tanks from day one rather than those who I have had to do regular 100% water changes on.