Let's say for fun that Snake wanted to keep doing his military-saving-the-world-super-soldiering, and had his remaining functional body parts merged with a cyborg body like Raiden.
Probably not how it should be, but he has come out of retirement at least three times already. Fighting is just his destiny. Why let a thing like accelerated aging stop him? We'll have to pretend he found a way to remove the Foxdie virus and stuff.
I imagine playing an MGS game where you are a rookie spec ops soldier and your CO is Old Snake, showing you the ropes while chain smoking and grumbling and dropping fanservice references to his previous adventures. Damn, that would be cool. Maybe he would change his code name to Big Snake or something lol