knackers for knees?
As someone who has had carteledge regrown in the knees, I can say that Aikido will not affect your knees as much as force falls of judo, or the striking mistakes of karate.
Once you learn to sit in seiza, somewhat comfortably, you will, in fact, think the years have melted away to a younger time ... that and the water pill I take for Meniere's pretty much took away all the pains from my early arthritus? But knees only get stronger with learning to practice Aikido, ask anyone who has more than a couple of years of practice can vouch for that?
If your weakness of knees persists, don't forget to support both knees, it really increases stress whe you don't use equilateral support, and healing is quicker, also. The fact is, my hips, knees, and joints actually clicked/snapped during the first year of Aikido ... all gone now. Go try a couple of months of Aikido ... don't forget to mention injuries if you have them, they will find ways to work around them. Have fun.