I have a mild ache in my knee and it makes a popping noise on extension, will i need a knee replacement?

No: Your "need' for a knee replacement depends entirely upon you and how you are feeling. Knee replacements are used for people with severe arthritis that have been unable to control their pain with a lot of different non-surgical treatments. The pain is typically severe enough that it interferes with their daily activities. After a good discussion with your surgeon, surgery can then be considered.

Probably not: The most likely reason for your symptoms is patellofemoral disease (kneecap/femur). There are many factors as to whether or not a knee replacement is needed, but, a mild ache would not indicate the need for knee replacement. A knee replacement is done for much more extensive symptoms than what you describe.