If you're referring to something like a Specialized, Fit, or Haro complete with a one-piece crank, that's exactly the kind of bike that you're going to find a kid riding. Go to your local bike shop and ask how many adults (or older teens) are buying complete BMX bikes. The number will probably be pretty low.

Anyway, to get back on topic:
if 3p=[3 piece cranks], and 1p=[1 piece cranks], then 3p>1p when 3p is not forged