The Health Fairies are waiting to start the B illnesses until you get into your 50s.
But seriously, have you had a lumbar MRI? It may turn out that you have something amiss in your lower lumbar spine that is causing the pain in your knee and hip, too. If there is misalignment in your spine, it can result in this sort of trouble.
Do you have any numbness or weakness in your leg? How long have you had the sciatic pain?
If you really think you are having pain in many joints all at once you may want to think about seeing a rheumatologist. There are some types of arthritis such as ankyosing spondilitis that need the attention of a specialist. Usually most of us have osteoarthritis in the spine...but sometimes it is more complicated.