@Digi-Lib hmmmmm... while I'm in the camp that does believe maturity (and at what age it happens) is very different from one person to another and that it doesn't necessarily change much past our teen years (like you've just said), i do believe some wisdom is gained an lots of spontaneity is lost just by the fact of spending more time alive. that being said, from the story events so far, i feel Koto is kind of mature, but VERY spontaneous and doesn't really think of the potential destruction that being caught in a compromising situation with her older boyfriend represents.
also, they're both kinda broken and they tend to make bad decisions. they're each other's life-boat, but it seems they might hit an iceberg soon.
i wonder though, i don't remember them talking about using someone with the same name to get out of this dicey situation?