I mean, both my parents were responsible for themselves financially and otherwise from the age of sixteen.
At eighteen, you're an adult. If you're capable of providing for yourself, your parents have no obligation to provide for you indefinitely. If you have autism that affects your ability to be independent, that's another story, but if not, eighteen is the minimum requirement for parents. Especially if your parents are getting on in years, it can be really stressful to continue to financially provide for an adult dependent.
If "help" means something other than financial aid, I think it's just a matter of parenting policy. There's tough love, and while we might not all appreciate it, some amount of respect is still owed to a person who stuck by their kid to adulthood. Many parents don't do that anymore. My own parents are far, far from faultless, but I have to acknowledge their efforts and be grateful for what they did that led to making me the person I am today, which isn't all bad. They usually did what they thought was right, even if they were wrong 90% of the time.
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I have not the kind affections of a pigeon. - Ralph Waldo Emerson