I just had a look at your horse profile - Lacy is 24, correct?
First up - 24 isn't too old at all :) Providing her joints can handle it, and her body and mind are up to being ridden, I don't see anything stopping you keeping her in med even full work. Including some jumping.
The great thing about using ground work with your work is it can help you keep your horse supple, flexable, and changes things up a bit.
Some things you can work on:
- Backing up (a true back up)
- Moving hindquaters
- Moving forequaters
- Moving both (side stepping over)
- Sensitizing (just as important as desensitizing!)
- Lunging, and long lining
- you can then start asking for more - shoulder in, shoulder out, etc etc
What ever you want to be able to do in the saddle, you should do on the ground first.
After a warm up (think of a lunging session - a proper lunging session (don't let her just run at her own pace, as she could do damage. Horses don't understand the need of warming up and cooling down.
After a lunge session, you can do some stretches (you MUST warm up before stretching - stretch a cold muscle and you're asking for touble lol). This is a good way to help keep your mares muscles moving, and the blood flow going. Also, you can 'train' her to stretch certain muscles (like you would a trick. I.e. tap her back leg and she'll stretch out).
:) just some ideas. My mare is 20 and I do most of this with her - she's also coming back into jumping as well, and kicking butt!
If you're worried about her joints, a joint suppliment may be a good idea. I like to use a natural suppliment, such as Celery Seed and/or Devils Claw. But there are many suppliments out there.
Good luck !