My friend just retired her gelding from endurance and he is 23, though she still rides him regularly. He's definatley not too old!
I would suggest working up the distance that you ride within a number of months up to half a year or more, depending on what distance you want to compete in.
The important part is getting his heart and breathing rate down within a certain amount of time after he stops and rests (can't remember off the top of my head, maybe somebody else will chime in) so you'll need to learn how to do that. There are plenty of devices that will record his heart rate for you.
Good luck! He's a cutie. (:
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