I'm going to cheat and say it depends. I have a Kabar large folder that my late Grandmother put the last edge on (Yes it is quite sharp) and which has lost about 1/3 of the blade to being sharpened over a great many years. I am not a Kabar guru but I would have to estimate this one is 50+ years old.
If however you count only a knife which I paid for, it would have to be the old deluxe Gerber Folding Hunter with the checkered French Walnut handle which I bought from Pasteur's Sporting Goods in Ocala, Florida in 1971, which makes it 28 years old.
Extremism in the pursuit of liberty is no vice, moderation in the pursuit of justice is no virtue.