I've had knee problems since I was 14, I am now 38, I am a candidate for a knee replacement but refuse it right now because even though I do have pain I'm not at a critical pain level that I want to have it done and deal with such a major surgery.
To be completely honest the decision for a replacement is yours not your doctors even if you are bone on bone.
If you can function daily, walk a mile (I'm lucky to walk to the mailbox

), play golf and water ski in my opinion no you do not need one.
You will need one if you can't ambulate well and have extreme pain. When getting out the bed in the morning is extremely painful. Mild even moderate pain doesn't mandate the need for a replacement.
My doctor has stated time and time again while I do need one the decision is mine to make, not his. Age isn't the factor I had my hip replacement at age 27 after several reconstructive surgeries. I can completely understand what it is like when you come to the point where a joint replacement is like.
I would suggest you get a second opinion and take your MRI and Xrays or reports to him or her and get their thoughts on if you need a surgery or not.
It does somewhat appear your current doctor is pushing you towards something you may not physically or emotionally be ready for.
The Arthritis Association did a study as well as research provening that 1200 mg of Vitamin C, preferably 1500, taken daily will slow the degeneration process and increases the time that passes by before you have a replacement. C is safe and disposed through the urine so there is no build up.
I follow this regimine and I can also say that it has decreased some level of my pain. I have bad days and then really bad days, very few good days but it has made a whole world of difference.
A joint replacement is a huge decision to make for anyone at any age. There are risks invovled and heal time can vary. Based on your post I can surmise you are an active individual who is probably fairly healthy so you would most likely recover well from the surgery unless you have a heart condition or something else that may cause further problems.
Your situation is not a life threatening one and if you feel you can manage a few more or even many more years with mild or even moderate pain then go for it, theres no need to ever rush such a major operation.
Best of luck
Barbie