How did Jimmy and Bob first get to know each other in "After Twenty Years" by O. Henri?

Jimmy and Bob have known each other all their lives. They grew up in the same neighborhood in Manhattan. That means they probably played together in the streets like all the other kids. They went to the same public school. When Bob is first talking to the beat cop, whom he does not recognize, he tells him about their long relationship, including the following:

I was eighteen and Jimmy was twenty. He and I were raised here in New York, just like two brothers, together.

This bit of explication is necessary to the story because it explains the rather strange appointment the two youths made to meet again at the same site in twenty years. Jimmy was twenty years old at the time. So he was making a commitment that was equivalent time-wise to his entire lifetime. How could he know where he would be--or what he would be? Bob was only eighteen. Perhaps it was because they were both so young and inexperienced that they were willing to make such an extravagant commitment. It is noteworthy that Bob, the adventurous one, had to travel a thousand miles in order to keep that appointment; whereas Jimmy, the conservative one whom Bob describes as "a kind of plodder," hardly has to travel any distance at all, because the site of their future rendezvous is right on his beat. Bob hardly had to worry about Jimmy showing up. Jimmy was there every night.

Although Jimmy and Bob were "just like two brothers," twenty years makes a lot of difference in people's lives. Jimmy and Bob could not be friends anymore. They are erstwhile friends. Their relationship is based on the fact that they used to be "just like two brothers." The changes wrought by the passage of time is the theme of this story. The whole city has changed, and the two men have changed--or evolved--correspondingly.