Winter can be a hard time for your golden oldies. While horses should be at their heaviest coming into winter so they can be slimmer coming into summer, it can be hard to keep the weight on older horses. Feeding mash can be a good alternative if they have dental problems to ensure they are getting their nutrients. Ensuring you have a warm rug on will stop the oldies/poor doers from using extra energy (thus losing weight) by just keeping themselves warm.
Arthritis can also be a big concern due to the cold wet conditions and potentially reduced turnout. Increasing turnout as much as possible is a good way to ensure they keep moving which will help to stop them seizing up. If field turnout is not an option, arenas are a safe way to allow them to roam about securely.