Re: Training over 50
This isn't directly advice; just a perspective on older aikidoka (from someone in their 30s.)
In our dojo, there is a wonderful person who began [began Aikido, his only MA experience] at age 55, and continues today beyond 65. (I think he's 67 now.) I started after him and passed him along way in the dan ranks, so he now listens respectfully on the rare cases when I have something I wish to say. But, to me, he still feels like a "sempai". He has some physical issues with arthritis and a slight disability, and tends to do ukemi with one preferred side leading. We don't throw him hard, and he does not perform as uke (as other dans do) during testing.
All that being said, he has a spark and life which show during his training. He can of course lead and throw. (In fact, I warn new students not to be mislead by his "frail" appearance.) His practice is earnest and honest. To me, he provides proof that size is not important in our practice. I hope I am lucky enough to train with him for many more years.