It's really about my feelings for me. If they feel like they could be 18 (in body & maturity), then I'm ok. But if they start to feel like a child, then I start to feel icky about it. Like Emma in Hillside, I can only see her as a daughter, not a lover, and won't be doing a romance path with her. Not shaming anybody, I just can't see her like that, but I know some people do.
In Boxed In, I have no trouble seeing Kelly as an 18 year old, while Grace is iffy (and I just avoided her).
With S&P, I also have no issue with Cara, she acts a bit immature at times, but overall, she feels legal age to me. Meanwhile Millie acts and feels well under 18, so, I won't be on her path at all. Mary felt ok to me too. I haven't met Hannah yet, because I haven't got to the latest update.