Rabbit Agility is definitely a real thing. Training is pretty easily and only takes a couple hours for most rabbits to be running a course.
As far as age for the rabbit, you can star them young but you want to keep the jumps low. A few inches for a 3 month old rabbit is good and you can go higher as they get older. They should not be going higher than about 12 inches until they are around 9 months so they get a good chance to develop physically. There really is no upper age limit as long as the rabbit is healthy and willing.
For equipment, a harness is good but not 100% needed if you are only working at home. Clubs and competitions require a harness and some need a leash depending on the environment (if the are is fenced they don't need a leash).
Jumps are super easy to make and don't even require power tools. You do need to have it so that poles can fall off easily and both ways. Some rabbits will head butt the pole and other kick it off, so it needs to fall both ways to avoid injuring the rabbit. Fixed bars should only be low, like a few inches. Other equipment can be made and it not too hard either, but can be a bit more tricky and expensive.
To train them, put the rabbit in front of a jump that is set low or even just a pole on the ground. Most rabbits have a spot around their bum or sides that if touched will get them to move forward. You can tickle this spot to get them to move over the jump. Give lots of praise when they go over. Keep doing this until they go over on their own. You can then raise the bar a bit higher if it was on the ground. Setting up a few jumps in a row gets them used to doing more than one jump at a time. Training contact obstacles (A-frame etc) is pretty much the same, but you want to make sure they don't try to jump off the sides. It is really just encouraging them to move forward and praise when they do what you want. If they aren't doing it right, like trying to go around obstacles, then you can use your hands to block them and keep enforcing them doing it right. Rabbits are pretty smart, so as long as you are consistent and give them praise, they will pick it up quickly.
I know that a few groups recommend placing the rabbit over a jump when training, but I really see little benefit in this. I think that the rabbit should be encouraged to go over the jump on their own even if it takes a bit longer, then they learn to do it on their own.
My rabbits won't take treats away from home, so using them for training really doesn't work. I have seen that rabbits who are trained using treats tend to focus more on the treat or lure than jumping the course. My rabbits and the other rabbits in the club are focused on running the course. They really run because they enjoy it. Even my new guy Felix will do a little binky when sometimes when he goes over a jump and he has only had a few training sessions.
