If they live long enough, many horses will become 'hard keepers'. It is a natural process: the ability and the desire to eat decreases and they can't digest what is given to them as well as when they were in their prime. Keeping weight on an older horse is limited by just how elderly the horse is for its age. One horse may become 'old' shortly after turning 20, and 'go downhill' quickly, while one managed similarly may be 30 and in good body condition. While you can only do so much, you can help most of the older horses by paying particular attention to nutrition.
First, as you probably have been told, older horses need their molar teeth inspected routinely, and floated so as level a biting surface as possible is maintained. By 20, many will have enough naturally occurring sharp edges and uneven biting surfaces that much grain is lost, and what is ingested isn't chewed adequately to help digestion.
Secondly, the aged horse should be fed smaller amounts of the daily grain allotments more often. Smaller amounts at one time will not overwhelm the capacity of the digestive tract, so rate of flow will be slower, and the potential to be digestive will be greater. A common recommendation for all horses is to split grain into two feedings when feeding more than 0.5% grain as a percent of body weight (5 lbs grain/1000 pounds body weight). When they become aged, it may be necessary to reduce this recommendation to 0.4% grain as a percent of body weight. If large amounts of grain are fed, you may have to feed grain three times a day instead of two to ensure maximum digestibility of what is eaten.
More so than that, what is fed has to be very digestible, and many of these horses may lose the desire to eat large amounts. Both of these factors led to the recommendation of using added-fat mixes for aged horses. Fat is highly digestible and very energy dense. Added-fat formulations will typically use corn oil or soybean oil as part of a pellet or as a light coating to a textured mix. Pelleted added-fat mixes are popular for older horses because manufacturers can mix in other feedstuffs into a physical form that helps digestion in horses with less than desirable teeth conformation.
Formulated mixes that are marketed to elderly horses will have fairly high concentrations of protein, minerals and vitamins. Increased concentrations help those horses that do not eat very much and horses that have lowered ability to digest feed. The rations also usually will have added-fat. It is easy to tell if a mix is added-fat, many will market products with labels indicating higher fat levels. If not, you can read the feed tag. Grains will contain 2 to 4% fat as a natural part of their makeup. When fat is added to the mix, the crude fat percentage on the feed tag will show levels more in the range of 6 to 10% crude fat. As long as palatability is not a problem, fat at these levels should add a readily digestible energy source without interfering with digestibility of other nutrients.
Finally, aged horses will need highly digestible sources of fiber. Hay and pastures have large amounts of fiber, and fiber should be part of the energy source of the horse's diet. Average quality hay or pastures with coarse stemmed plants or mature grass will not be digested very well. On the other hand, well managed pastures with green, immature growth of improved grasses such as bermudagrass and wheat will have more digestible fiber. These type pastures supply a continuous supply of readily digestible energy, which with some elderly horses, quickly increase body condition.
At times that pasture forage is unavailable, the horse will have to rely on other fiber sources. Many feed manufacturers have developed 'complete feeds', which add a ground source of fiber to the mix so what is found in the bag can be most, if not all, that is fed. Alfalfa cubes are also an adequate way to supply high quality fiber, and there may be some digestive benefit to ensuring intake of some minimal level of fiber at least the size of the hay particles in cubes. Cubes may be too hard for horses with poor teeth, but this is easily handled by pouring water over the cubes several hours before feeding. This will make the cube is softer and more easily chewed.