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Wow, that's an old snake! 20 years is not unheard of though. But I would definitely say feed it larger meals, and f/t. Aim for something that is about the size of his widest point every 3 weeks (probably something like a medium rat), but start out easing him into larger food. Ball pythons can be picky about feeding changes, so take it slow. If I were you I'd start out by getting him to take a few f/t mice, then after a little while switching to f/t weanling rats, then small rats, then medium rats (or whatever looks like it's an appropriate size). The key is to take it slowly. Overfeeding is more dangerous than underfeeding.