At age 55 I took a bone density test and discovered I had osteopenia. Within months I had broken two toes. You may find that you can’t hit anybody for fear of breaking your own bones. I took Fosamax for several years and extra calcium and got my bone density close to normal. But there are side effects.
Believe me, I’m in good shape for my age. Hardly a day goes by that somebody doesn’t comment on the bulging arms or six pack. (Okay, mostly it’s me who comments, but I am well developed and have a flat stomach.) But it would be crazy to hit somebody hard point to hard point. If you must go with martial arts, stick with Judo.
The point is, you need a weapon. Go talk to a martial arts person and get a recommendation for something you can carry. Two heavy sticks with a short string. (Get good with them.)
I still have my extending baton. It’s nasty at full length. I also have a PR-24 from my Navy days, but never counted on it on than for self-protection.