My friend owned a 45 year old pony that I rode once :P. He was an awesome little guy, tons of attitude - if he took offense to the farrier he could throw the him into the wall. It was downright impressive.
I also used to trail ride a wonderful 33 year old arabian gelding. He could go w/t/c and would have gone all day if you would let him.
There was also the 30 year old ex pony club lesson pony. He was a full welsh, and quite the show pony back in the day. He gradually became just a beginner pony, but still liked to go out for a rip... I remember galloping him once and wondering why it felt like he was about to fall over, turns out he was bucking the whole way :P. He was put down just over a year ago due to colic at 35.