Exercise and knee pain

[stormkeeper_lovedoris]

I do a cardio workout almost every day. Sometimes I wonder if there could be negative effects on my knees over the long run. I was surfing through a discussion group on exercise and I just remember one person writing "Your knees won't last forever". I am careful to stretch before and after workouts, to get new aerobic shoes regularly, to limit step aerobics to twice per week (I'd love to do step more often), and to do exercises to strengthen my quads. And my knees do feel just fine 99% of the time. But still, I wonder if there's something different or something more I should be doing to make sure my knees last as long as possible.