Totally depends on the situation. After college when I was saving up, my parents didn’t charge rent. I lived with them for a year and was able to get established. I was pretty motivated to get my independence. If I had a kid that needed some motivation to be independent, I would consider rent as a little push.
My parents let me and my siblings live with them until we got married (although I was away for schools for many years) and even helped me with down payment for my first condo.
I believe their generosity didn’t spoil me but helped me greatly.
However, they made two things very clear. They said they would neither babysit my children nor give/leave any money beyond that down payment.
I want to offer the same thing to my kids if possible. 
What happens if it is the other way around? Would you like to be told “hey dad, mom, got to come with the rent dude!”
Interesting stuff, @Jane…so what is their expectation on care for them in their senior years? Do they expect you to do it or is assisted living ok with them? My siblings and I are doing it, on a rotation basis, and it can be tiring and challenging…
Obviously in this era of high housing costs and millennials with possible huge student loans to pay off won’t the “dependence” on Mommy and Daddy be way more pronounced in time and amount financially?
Luckily, they are still healthy and financially secure.
They always said that they would move in to senior home (assisted living) if they couldn’t live independently without help any more.
However, I hope I can take leave from my work so that I can help them when they need my help.
My siblings live close to my parents and help them when they need supports.
I guess I can only tell when hard phase of life finally arrives at our family.
If quid pro quo is strictly followed, then shouldn’t senior years care be expected as a return for the 18yrs of birth-to-college support parents provide to each of their kids? And, without belittling the huge physical and emotional toll of caring for elderly parents, is it more trying than what they went thru caring for us when we were infants?