To be honest, I paid little attention to my son's penises after they were out of diapers. We're not uptight about nudity; we just all prefer to be clothed, and always have. DS1 used to "adjust" himself alot, but neither boy was seemingly preoccupied with his bits.
I know DS2 was retractable by the time he was four or so, because I was helping him wash his hair and noticed that his foreskin looked irritated. I asked him, "Is your penis a little bit red?" He retracted, looked at the glans, and said, "Nope....it's purple."

I have no idea when DS1 retracted. I'm assuming he has by now, since he's nearly grown.
