I agree with Geothe

If you are going to allow multiple classes, the deciding factor in most competitive sports should be weight, height, or some other physical measurement, not age. Let like compete with like.

I would make an exception if someone ever came up with a sport where sheer experience in life on this planet was a factor. In those cases, there would be a certain age (perhaps young, perhaps old, depending on the sport) that would have a natural advantage.

There are some sports where, for amateurs without sponsorships, youth has an advantage not because they are young but because they have more time to practice: Adults work 40 hours a week and have other responsibilities, kids go to school 30-35 hours a week and don't have as many outside responsibilities they can't avoid. This leaves more time for training. You get the same scenario in most universities: The underclassmen have more free time than upperclassmen, who have more free time than graduate students. Unless the adults (or older students) find a way to make time (such as getting sponsorships so they don't have to have a day job, or find a way to get paid for doing their sport, or cutting their class schedule), they will be at a disadvantage.