Get started now. I don't know how bad your knees are but learning to care for a horse could be accomplished without riding and at a barn that perhaps caters to older riders you may find that they are set up with ramps or taller mounting blocks. Starting an exercise program would help as well but just being on a horse and walking is get therapy for lots of ills and shouldn't bother your knees. As for medicating and feeding every situation is different. Horses should always have free choice water but I have been in barns that water was dished out in 5 gallon buckets. Not the best but they would get fresh morning and night and offered after a workout once cooled down. Different horses have different needs some drank it all and we;d have to put an extra bucket in others never finished a bucket.
Two of my closest friends in Tx were quite a bit older than me (and still older than you) with various limitations. We all rode and drove. One of the friends had made the switch to a lovely little (14H) Paso that already had the basics down as far as riding but still considered green as he didn't have the miles. What he did have was a steady reliable nature. We taught him to drive over the summer for two reasons - one she loved to share and it is far safer to drive your own and include your friends then have an extra horse available and trust that someone won't overestimate their skill level. What made this choice perfect for her was that he was super smooth (gaited), close to the ground and willing to do anything she asked. In the end she bought a second Paso with much the same personality and did the same with him. He was a companion to hers and there for those that did enjoy riding and were capable as most of her friends were horsepeople. You can find smooth non gaited horses but you have to do a lot of trying before you buy. You really should do this anyway. My other friend went with haflingers as she was more inclined to drive than ride and most of her friends were not riders. They were also smooth and comfortable to ride for when she just wanted to go out alone. In the end she too bought a buddy - another haffie.