I was just about to say, a popular 1980's way to age 501 blue jeans and blow out the knees was with sandpaper.
First you would put them on, sit or squat down and lightly sandpaper one or both of the knees, then the pant leg hems and outer raised seam edges - making sure to avoid sanding/breaking/stressing any actual seam stitching, you want to try and maintain the integrity of your garment. After that, a couple good heavy washings in hot water, accompanied by a little Clorox and you're ready to hit the Palladium and go see the Ramones. . .