Yeah, I'm going to go against the grain here and say…18 is old enough. Obviously, it's wonderful if parents are financially capable of supporting their children past that point, but often-times 18 is a good age for the kid to be out of the house because it then allows the parent to start putting the money they're earning towards their own retirement rather than towards their kid, typically with just enough years left to save up some sort of a safety net for themselves.
If we made parents responsible for their children indefinitely…what motivation would kids ever have to leave? From what I experienced living in Japan, parents have a tendency to allow their children to live at home (since rent prices are absurd) until they either pass their mid/late-twenties or get married…and from what I've seen (and this is obviously personal), they use their money to buy themselves expensive toys like cars, televisions, etc., and rarely contribute tot he household expenses. In fact, they're more than happy to let their parents do the laundry, cook, and clean, allowing them to use their money for purely personal reasons (all the while rationalizing that they're 'saving money').
I 100% agree with Tairis…at a certain point, you have to take ownership of your life, no matter what led you to where you are. If that means getting a job that pays the bills but isn't your dream job while you try to figure other stuff out, then that's what you do. And roulette, I'm sorry you went through everything you did - but I don't think that's the typical situation in the U.S., and I feel like making a law based off what is likely a small percentage of the population wouldn't be particularly beneficial. Would you argue that your parents should support you until you decide you don't want them to (even if you're, say, fifty)?
Obviously parents are obliged to make sure their kids are able to get through their childhood successfully, and build the basic blocks they need to move into a successful adulthood - and many parents continue to support their children through college, if they choose to go. Parents who don't do this should be punished, and we have a legal system that, while it sometimes does fail, is generally set up to do that.
I just think that forcing a parent to support their children beyond 18, legally speaking, would end up having unintended consequences where we would see people having less money saved for retirement, more people living at home and spending money on personal pleasures while their parents provided on the basics, and a general push towards kids becoming ready at a much later age rather than at 18.
Just my two cents, though.