I am 51 now...I have also been on Military Disability since a bad jump (Parachute Accident) in the late 80's with left me with a bit of metal in my body.
I have to work out hard off the mat in order to maintain a good base for practice...
On the mat I am more relaxed and softer so actually Aikido is easier physically in allot of respects except breakfalls (because I am just a big dude)...Still... there are always a few bad tweaks here and there because of a poorly executed technique between usually a newbie and I... when I allow them to muscle through something and they try to jerk my arm out of my shoulder or hold on a split second too long with a throw.
Practice with a heavy Bokken (suburi) is of great benefit too as well as really focusing on breath control and using my entire center and RELAXING no matter what... Especially under physical duress...To much adrenaline makes your muscles more susceptible to injury in my experience. This aspect of practice...learning a more holistic approach... May go a long way towards prolonging my Aikido life.
As I progress I see the dire need to bring real Aiki into Aikido...It's health benefits from what I have seen are amazing. I met a 65 year old woman Chinese IMA practitioner that handled me like a rag doll earlier this year and she told me because of "Chi" she have not been sick or hurt in over 20 years!