Context of "when?"

Question

One example comes from this song in "the Music Man."

Gary, Indiana, Gary, Indiana
Gary, Indiana, let me say it once again
Gary, Indiana, Gary, Indiana
Gary, Indiana, that's the town that knew me WHEN.

Another example comes from a friend who met Taylor Swift at age 13, before she became famous, and therefore knew her "when."

The meaning of "when" is something like, before one became established or famous.

But what is the context/appropriate expression of it? Is it something like "when I was young..."?

(On the other hand, if someone is young AND famous, then it's no longer "when.")

Answer 1

"I knew [him/her] when" is an idiom in English. It is not about the speaker's age (I knew her when I was young is not an idiom.) It is more about a defining event or series of events in the life (no matter how young) of the person about whom you are speaking. One dictionary defines it as

At an earlier and less prosperous time. "He's mister high and mighty now, but I remember him when."

On May 22, 2007, a video of a young boy and his younger brother was uploaded to YouTube. The young boy (Harry Davies-Carr) is holding his little brother Charlie on his lap. Harry explains that Charlie bit him, and puts his finger in Charlie's mouth to demonstrate. Charlie bites him again, to no one's surprise. Only this time it hurts!

The video went viral, and has been seen over 720 million times. Harry and Charlie became minor celebrities. Though Harry was a young child when the video was uploaded, anyone who knew Harry and Charlie before the video went viral can rightfully claim:

I knew Harry and Charlie when!

^{The video is really cute. If you haven't seen it, do watch it.}