>Males almost never ever use this regardless of age, unless engaging in feminine play as parody, and it gets dropped among females cognizant of their perception as adult women retaining possession of responsibilities and authority.

I'm a heterosexual male in my mid-20s and many in my gender and age bracket say "kk" and "ooo" online as casual responses to things all the time - including myself. I wasn't aware we were teenaged girls. I don't think either word is inherently feminine, cutesy or immature; just very informal.

The thing I'd associate the most with younger American girls is frequent use of ellipses, often (but not always) with 1-2 less or more dots than is required. Like "k.." or "nice....". I think this is to some extent emulating the "high rising terminal" vocal pattern commonly associated with American women: https://en.wikipedia.org/wiki/High_rising_terminal#Implicati...

And of course emojis/emoticons.