Skipping per se, at your age is not bad for your knees but you do have to consider a few factors - Do wear a cushioned pair of shoes when you skip so as to reduce the wear and tear of the joints.
Do skip on a soft surface like a mud track or a field instead of hard ground or the floor. This will again ensure less wear and tear.
If you are overweight, in the long run, you might just be increasing your risk of getting joint problems, not only in your knees but also anywhere in your back, shoulders etc.
So you might like to alternate skipping with brisk walking or any other aerobic exercise to get the same benefits without the risks.