I wouldn't say that Dark Sister is looking down on people younger than her (not being able to relate to them isn't condescending). I don't understand that interpretation. Can someone please explain it to me? (No snark, I just don't see it and am trying to understand.)
I also think the reason Dark Sister feels slighted is because of the tone of voice in which FSIL is saying it, as if Dark Sister should feel like an 18 year old in the wake of FSIL age/experience. As Merry Mrs Martin put it, it's a matter of delivery/context.
I'll be very honest and say that, at 24, if someone told me I acted like I was 18, I'd be insulted. I am way different than I was when I was 18 and that all came with age, the maturity, the outlook on life, et cetera. If someone told me I was just a silly college freshman, I probably wouldn't have anything nice to say. Then again, I'm insulted when people call me "Hun" because I think it's belittling, which may be causing a bias in my opinion.