I'll be 55 in a few days, so I think about this a lot too.
Haven't come up with any good answers, other than to keep training and do what I can. So far, I can still take all the ukemi I expect my students to. At some point I may have to cut back on that stuff, but I reckon that's part of discovering your own Aikido.
As I frequently remind my students: Age and treachery are an even match for youth and enthusiasm.