Pressure points rather than punches and kicks. Slipping and parrying in preference to brutal blocking. I like joint locks and disarms
I'm not a big fan of pressure points as many schools market them. Too many schools market pressure points as if it's a button that you can simply press and the attacker will "shut down." I use pressure points in school but they require conditioning which is often feels brutal but is needed to build up the bone in the body parts that we strike with. It also helps us to learn how to better target the pressure point.
Slipping and parrying is taught in most systems. "Brutal blocking" is usually what people say when their arms aren't conditioned. Martial arts systems in general don't preach to meet force with force when it comes to blocking, unless they are trying to break something or jam something.
From a health perspective since you have bad knees. I would probably recommend a martial art that will actually help to strengthen the ligaments and tendons in the knee and other joints. This will benefit you as you continue to get older and will go a long way to helping you remain mobile (with the exception of disease causing the problem)
To refine my original question, it's not really about who hits first, it's about appearances.
If your reaction to someone else's aggression is to take up a stance:
1) You are inviting, arguably even consenting, to violence.
2) Tipping off the aggressor you've had at least some training.
3) Making him look a chicken in front of his buddies if he backs down, meaning violence is almost inevitable.
4) Looking less sympathetic to a jury.
1) I don't like anything these 4 things. If you are being harassed, in a heated argument, or any position of where you feel like you need to defend yourself, then that "Invitation" was not only sent, but the person accepted and showed up for the party. I don't know why people have this "Keep it a secret that I'm about to defend myself" mentality.
2) I want my attacker to know that I have had training. I actually want my attacker to know this even before he approaches me. I want him to look at me and be uncertain about his outcome if he attacks me. Uncertainty prolongs decisions, so if I can keep an aggressive person in that state of mind then it's better than me. I don't want to look totally helpless like a victim. Looking like a victim is an invitation to violence. I also don't want to walk around like I'm the Alpha man that dominates over everyone. This can attract violence just as easily. The problem isn't letting my attacker know that I have training. The problem is letting my attacker know the extent of my training.
3). Making him look a chicken in front of his buddies if he backs down. This is a personality issue and not a martial arts one or self-defense one.
4) Looking less sympathetic to a jury. I wouldn't bank on this one. If you are think you are about to get into a fight then, always try to position yourself as you were the one who was trying to avoid confrontation. If cameras are around then stay in front of the camera. If witnesses are around draw attention to the conflict by talking louder and literally saying "I don't want any problem." "I don't want to fight." Layout as much as a defense before the fight actually starts. That way you don't have to try to count on someone feeling sorry for you just because you took the first punch to your face and as a result ended up in the hospital.