I don’t think maturity is an explicit thing in a binary form, i would be ok with the presumption that the age of 18 provides a general expected range of maturity between individuals, it’s when you start to develop your world view and really pick up on the smaller things in life and how they work together to make a functional system.

I think the idea of putting a “line” on it, is wrong, i think it’s better to describe it “this is generally what you expect from this subset”