Joints in the body like Shoulder, Elbow, Hips and knees are continuously under friction from movement, to facilitate this movement smoothly body produces a oil like fluid called Synovial fluid which lubricates the joint and helps is smooth movement between the surfaces.
With increasing age or due to other many reasons this synovial fluid and the cartilage lose their ability to regenerate and repair themselves leading to slow onset gradually progressive Arthritis/Joint pain.

Non surgical-
Decrease in Body mass index or in other words weight reduction- this is the ONLY SCIENTIFICALLY proven way for nonsurgical treatment of Arthritis.
Others like Hyaluronic acid injections, PRP injection, Nerve ablation to make the patient not to perceive pain are available but are of questionable efficiency as per literature and studies
Surgical- Total or Partial knee replacement.

Joint replacement surgery is wherein the arthritic joint surfaces are replaced by a metal prosthesis with a highly crosslinked polyethylene in between for a smooth gliding movement again.

This is a very important question.
Any person who is unable to do his day to day activities because of pain arising from the joint is for Joint replacement.
Day to day activities like getting out of bed, difficulty in walking to the washroom, wearing clothes, walk for even short distances.

In an appropriately performed knee replacement surgery with Implnats which have been tested under Lab conditions and have given satisfactory data, lasts for approximately 22-25 years.

You need to discuss about these with your doctor for a better understanding.

Yes, it is for an untrained mind and body.
If adequate PREHAB, which includes Physiotherapy comprising of knee exercises, acceptable control of other medical illness like Diabetes, Hypertension, Thyroid disorders etc, will help in reducing the pain to minimal or bearable level.