Hi Cara,
I, too, have a 20-year-old (she doesn't act it, sometimes) and I've found that she has tended to be too skinny over the past couple of years. Her backbone starts to show, her butt gets hollow, her belly sags (hey, mine does too, and I only had 2 babies, compared to her 10 or so!), and her hip bones get visible...
If this sounds like your China, you might try increasing the protein in her diet. With Crystal, I've added alfalfa for protein, and switched her to a high-fat feed, and she's doing soooo much better. I think she actually had some ulcer issues, wouldn't "clean her plate," and was fussy about food. The alfalfa seems to help that a lot - I give her a double handful of chopped alfalfa morning and night with her grain (she also gets 1/2 pound of high-fat feed - Poulin's Pro Max 12 - twice a day and free-choice grass hay) and she's cleaning it all up now and has lots of energy.
The other thing to think about is exercise. We mostly just walk our guys, a couple miles up and down the road, and even that makes a difference in how they look and act. Ours are looking so good right now that I'm tempted to enter a local show or two... We do lunge some, also, but not more than 10 minutes at a time, mostly trotting.
Good luck!
Lori
PS: I agree with rabbitsfizz that you don't want to do anything too drastic! Make any changes slowly, over the course of a week or two, and see how she's doing then.