I was on the Standard American Diet, just like you... and like I say, I didn't start to develop problems until I was in my 30s. I think it's pretty uncommon for younger people to have buildup. BUT never too early to prevent!!!
And what makes you THINK you have buildup, anyhow? Like I say, is your stomach... protruding (as mine certainly was when I started serious colon cleansing three years ago--I thought was fat!!! imagine my surprise!!!) Or when you have BMs, are they pencil-thin, or rabbit-like, or weird in any way? This is a good indicator they are passing through a constricted area (i.e., a colon with lots of buildup on the walls)... What symptoms do you have that cause you concern?