I successfully indoctrinated 2 of my 3 kids by 18 months. My middle child is too much in to building things, tinkering with things, and the like to get much in to music or audio. In a couple years I'm going to buy a small speaker kit and we'll build it together and those will be his bedroom speakers. Maybe that will get him in to audio. :)
My oldest already comments on the difference in sound between the home stereo, the computer, and the car stereos - and started doing so at age 2! :) Needless to say, I'm a proud papa. :)