First of all, let me say that carrying a 90 lb. pack and only tipping the scales at 115 is a crazy crazy thing. That is certainly overkill for most and has probably contributed to some of your knee trouble, especially if you made a habit of it. When I back packed when I was younger, I regularly carried a pack that would weigh close to 60 lbs. and that was all I wanted...period.I am getting close to the same age as you are (oregonmouse) and I carry around 23-26 lbs., depending on what I am doing and where and when I am going. Two liters of water with that gets the weight up about four more lbs. but that's the nature of the beast as far as carrying water. Some times you just have to do it. If I go to water plentiful areas, I usually trim down a little on the water but not much because I drink a lot of water on the trail. I am noticing that even at my house on the stairs that my knees have taken some wear and tear as well. They still do pretty good but when they are stressed, I can certainly tell that they are....sabre11004...