They just want to be as cool as their dad was back in the day. Or maybe neighbor's dad.
The thing is... Dad's Harley didn't cost 20K. It was probably a government surplus bike bought on the cheap, had straight pipes because dad couldn't afford factory mufflers, and needed to be revved up because dad was trying to squeeze some go out of that heap, and maybe wasn't as good a mechanic as he thought he was.
A lot of stuff like that was born of necessity and became part of the image. I bet more than a few of these guys don't actually know why they do it. They just see it as a "Harley guy thing to do".
They are older because they finally can afford that 20K Harley at this age.