I agree 21 is not "fragile". But, this mare has had a pretty demanding career, if she truly was a "national level" show jumper, and then a school/lesson horse. You will need to monitor her closely for lameness and arthritic issues. If your trainer is a good trainer, she will also be aware of those possible issues.
The mare might require more expensive and/or invasive joint supplements, treatments or injections. She might also require more warm-up and cool-down time to keep her from getting stiff and sore. You might want to research wrapping her legs, or using boots, to give her more support when ridden.
If she hadn't been a show jumper, and had instead been a pleasure horse, or even a rail show horse, I wouldn't be as concerned about her joints being damaged...