I think longer hair makes him look older, in addition to the lines on the face and wrinkles that have arrived (starting with Pop). He's had long hair for most of his career, why not give another haircut a chance?
Consider men DO lose their hair and go bald, it's a natural process. And they DO get grey hair, like Adam's. Which I don't mind. Larry's had very short hair since Pop, too. And Edge, from what we've seen on the hatless pics posted, is basically almost bald.
Why is it that important to some how his hair looks like and why can't anyone ever say anything critical about Bono's hair without getting flamed? BTW people don't look at someone's hair do determind their age, but their face.
(remember the hair implant comments during the last tour? the longer he keeps it up, the more suspicious it's gonna look and he's gonna become the butt of jokes)
I think that the ultra-short shaven Pop look was too much, but so is what he's wearing now. (the color makes it look passable, much better than the reddish color he had for the past two years) I think he looked younger with the BD/ES haircut, or at least shorter Elevation hair - though even that, once he sweated it out during a show, looked odd.
There's plenty of famous bald (or almost bald) men out there that prove it's not the end of the world: Kojak, Sean Connery (did you know he was voted sexiest man? sporting his current look, mind you), Michael Jordan (ok he shaved it, but still), and from the world of music, Michael Stipe.