Wow, that is a clean, low hour tractor to be in the 20 year old range!
I guess you will get a lot of opinions, so I will only offer mine- I'm not an expert.
While I believe in maintenance, I don't change something just because of age. If it still works, and doesn't show signs of giving out, I keep using it. If it were my tractor, and the oil in the axle and transmission look clean, don't show contaminants like water or other floaties, and it doesn't smell burnt, I would keep using it. Again, that is just my opinion.
I would never do this to my equipment, but an elderly neighbor who passed away about 6 months ago had an 86 Honda Foreman 350 ATV. This old gentleman could break an anvil- he abused everything he owned. That atv still runs strong, and he never changed the oil in it. It still has the factory oil. Maybe that's Honda for you, or maybe oils hold up longer than we give them credit. I believe that in this case, it's a little bit of both.
Welcome to OTT, by the way!