responsibility over time

I have a sizable database of all my school stories (which I collect, as you might know). Occasionally, I go on the fence about whether to include the Private series, or Ally Carter's series about the Gallagher Academy, or the Chestnut Hill series, or the Royal Ballet School Diaries. All of these were written in the last five years, but they're not exactly Girlsown. Ultimately, I threw these recent books in, but I haven't been happy about it.
I realized tonight that I haven't had much worry about including the books about younger girls (Royal Ballet School Diaries and Chestnut Hill). I think that's because these girls are young, so it's not much different than Clare Mallory's books about Merry (written in the late 40s/early 50s), or Brent-Dyer's or Blyton's books that focus on young characters (written between the 20s and the 1960s).
But the books about older people (I mean 17- and 18-year-olds here) are different. In books published before 1970 or so, it was assumed that by the time you were in the 6th form, you were sensible and responsible and self-controlled and things. (Of course, this wasn't the case in real life, always, but that's not the point here). In contrast, Reed in the Private series, or (good heavens) Jenny Humphrey in the It Girl series are fairly irresponsible.
Of course, they can be. When they're 18, they'll go off to college, and then they'll start working out what they want to do with their lives, and who they want to be. But girls from an earlier generation - basically, girls in most Girlsown books - start this process three or four years earlier.

P.S. For the sake of my reputation, I should mention that I don't own any It Girl books. I mean, enough is enough. Besides, Jenny's one of the most boring characters in the Gossip Girl books, so why read a spin-off about her?