Board Thread:Questions and Answers/@comment-24520051-20171203215353/@comment-1257201-20180519023718

A flashback would count, as long as the character was a costumed hero/heroine in high school. So a flashback to Bruce Wayne's high school graduation doesn't count, not because it's a flashback, but because Bruce Wayne wasn't a costumed hero prior to graduating high school.

But whether or not it's a flashback, I'm looking for the first in publication date. For example, the Earth-Two Robin first appeared as an adult in (August 1967). If his graduation had been mentioned or shown in a flashback in that issue (which it was not), then it would count as August 1967, even though theoretically that flashback might have taken place earlier than that.