No, your responsibility as a parent doesn't end when they hit a magical number of years. You chose to be a parent, that's a lifetime commitment, not an eighteen-year contract.
Im 42 now and lived in a very small house with parents who worked very hard and struggled with money lots of times but they never asked me and my siblings to pay rent. children living with their parents as an adult should be a given. Everyone does it, nobody thinks lesser of young adults living with their parents. Certainly nobody charges their kid fucking rent to live in the same place they have their whole life.
Of course, I don't mean you let your child leech off you till they're 40, never working or doing anything worthwhile with their life rotting away in your basement. But, provided that they are actively making something of themselves, be it education, pursuing their passions or something that will eventually lead them to achieve full independence and move out anyway, adding an unnecessary expense to their budget just makes the time they take to get to that point even longer.