User blog comment:Ultimate Heatblast/Wonder Girl and Superboy/@comment-5324323-20120729182848/@comment-1038387-20120729200914

As for Cassie's experience, in "Beneath", it's pretty clear that she's the least experienced of the four ladies. In "Depths", her behavior similarly shows she hasn't been training with Wonder Woman for long. Based on the lineup and additional information we have of "Young Justice: Legacy" so far, we know she wasn't a member by that time (December 2014/January 2015).

My guess is she's been a superhero since December 2015 at most, and we'll be seeing her origin in the coming 6-parter comic.