NKC,
Unfortunately, with so little information it is not possible to determine what is causing the discomfort over the internet. There are many, many causes of pain in the knee. Depending upon where, exactly, the knee is hurting, some of the more common causes may be:
Degenerative joint changes in the articular cartilage are a very common cause of knee pain. This usually begins to bother people at about your age. If the patient has had significant injuries to the knees when younger or they have a family history of osteoarthritis, this is a very likely cause.
Tears in the meniscal cartilage can also set up inflammation in the joint, which usually results in knee pain. The menisci, again, begin to degenerate about the 4th or 5th decades of life. A meniscus can be torn from an injury or just due to aging. The type of injury that usually causes a tear, is a twisting type of injury.
Patellofemoral syndrome (PFS) is a disorder that causes pain under the patella (knee cap). These patients complain of pain after sitting for a while, climbing ladders, walking down stairs. Occasionally, the patients will also have some swelling under the patellar tendon.
The are a lot of tendons and bursae around the knee with can become inflamed. This can thing cause discomfort with certain activities.
But, the only way to know for sure what is going on, is to have the knee examined by an orthopedic surgeon. Usually, screening x-rays are taken also. If necessary, an MRI can be obtained for further evaluation.
Good luck.