I have no idea what fully grown or fully mature mean. Fully grown, must be a scientific way to plot maximize size out?
Fully mature, I have no idea what that means...do you mean sexually developed or "has a clue"? As everyone knows, there are people that literally live their entire lives without "maturing" in the adult sense. But compared to who? What's the point of the question?
There is absolutely nothing about goal setting and achievement (project management), or personal finance (addition, subtraction, multiplication, division, percentage, and an excel table) that requires anything that a 15 year old shouldn't be wholly prepared to master.
By 18 you're basically treated as an adult. Kids leave home, they often drink, start families, start careers, etc. If you don't want to prepare them for that, that's fine. I think it's appropriate to give them the tools to help them navigate that, more so than I think we should teach them to memorize the presidents. Just saying. And if they don't care, they have to pay a price. You can't insulate them from the negative consquences of refusing to be responsible. That in itself I consider to be irresponsible.