Lots of people fight full contact (muay thai, kickboxing, and full-contact karate systems like kyokushin) for years and take blows like this regularly - if the guys got up at the time and seemed ok, the chances that your blows had any significant long-term effect is incredibly small. That one of the guys was still standing suggests you didn't get a ...
I'm over 40 and train in taekwondo; I've never been advised not to do jumps or jumping kicks! I train with a gentleman who doesn't jump (for patterns or sparring), but he is nearly 80 and has arthritis!
That's not to say my knees don't hurt at times. Some years ago, I had a problem with my knee called chondromalacia patella, which is caused by an imbalance ...
I have no experience with elastic bands - so hopefully someone else will pick up on those.
I have tried Knee braces - but they often restrict the jump motion and the less restrictive ones are more prone to moving and less useful in protecting the knees.
So my tips for practicing those jumps are as follows
Find a sprung floor - this will allow you to ...
Here are a small list of styles to look into. They're all hand dominated and have forms:
Wing Chun kung-fu
Southern Praying Mantis kung-fu
Hung Gar kung-fu
Taiji (Tai Chi)
Many other styles would qualify.
For example, I listed a few southern styles of kung-fu up there. There are a lot more southern styles than that. And they do tend to be ...
Land softly and work up
For those with the patience, I advise starting from a soft landing and working up from there. When you land, you reduce the forces on your joints by decelerating over distance. Land starting with bent legs and decelerate as your legs bend more. This is similar to catching an egg; you give to prevent shatter upon impact. I normally ...