If all other physical reasons have been ruled out, then you will want to work on feeding and light exercise. Horses that are aged can be harder to put weight on, but I was recently a large part in bringing a late 20s mare from your horses previous condition to a healthy weight, so it is very possible.
Alfalfa cubes are helpful. We gave this horse alfalfa pellets 3 times a day, 2 qts each feeding. She was also started on a Purina supplement called Amplify. I would definitely recommend it.
Next, I would actually start exercising her a little. Horses in extreme starvation mode are hard to put weight on. I would start handwalking her, starting with 1/4 mile a day and working up to about a mile a day, more if she is handling it well. We aren't just looking for adding fat, but also muscle. Your pace in the walk should be steady, not too slow, but not rushing either. Grooming is also very important. A good daily curry and brush especially along the areas where there is supposed to be muscle mass will help to stimulate the area and improve circulation.
We often times think too much about the best way to gain weight is sit on the couch and fill up on food. That is how we gain weight, but not our way to better health. Not to mention, all of the hand walking is good for the person too! You will be surprised that with regular maintenance not just into what is going in the body, but what the body is doing will help the horses whole overall appearance. Good luck!