There really is no upper age limit as long as the rabbit is healthy. Korr started when he was almost 4 and is still going strong. He does have days when he is not as in the mood for it, but other times he still races around and jumps high, he is 7 now. He was not too impressed the first time I tried with him, but once he figured it out there was not stopping him.
Start with a low jump, a couple inches maximum. Have a pole set up so that it will fall if she were to knock it off, you can rest it on some DVD cases or books to start with. Put her in front of the jump and use your hands (never use your feet or other objects to get your rabbit to move) to direct her to go over the jump. Many rabbits respond to a light touch on their bum or sides to get them to move forward. Once she goes over, give her lots of praise. Keep getting her to go over until she understands that you want her to go over the jump when you put her in front of it. Once she understands the concept, you can increase the height. Don't go too high at first, she needs to understand more and get used to jumping first. It is a good idea to have a few jumps set up in a row so she can get used to jumping more than one jump rather than just one and stopping.
It is a good idea to use a harness when training. If there are any clubs in your area and you wanted to compete, you would need a harness for safety. A leash can be used as well, but I feel it is better to focus on off leash work and only use a leash if needed. A leash should only really be used for safety (you are in an area that you rabbit could run off) and should not be used to pull the rabbit around. An H style harness is best as it is safe and not too bulky on the rabbit.
Most rabbits learn quickly, but since you are also learning it may take a bit longer. Don't get too discouraged if she doesn't pick it up right away, try again and keep trying. Lots of praise and encouragement go a long way.