My answer for both of situations (parent with child, child with parent), is likely to be the same. Although, I would give more leeway to a person attending some form of college or vocational training, regardless if that was a traditional student or a returning-ed adult.
I don't think there is any moral obligation for the parent to pay rent to their child. I also don't think the child has reasonable cause to just outright declare a rent value and expect the displaced parent to pay it. (Since I don't think there's specific moral ground for the child to charge rent, I would say it's immoral to do so.)
I will concede that there are certain costs that may go up, on account of having additional household members, that can reasonably and morally be expected to be paid for. For instance:
- The increase to utilities, if any
- The increase to food costs, if any
The parent may be buying their own food anyway, especially if diets and tastes differ. As for as utility increases, I would suggest some method of payment, by money, goods, or services, to only be to cover the changes directly attributable to the extra person. For instance, if the water bill goes up $20 a month because of an extra person taking showers and doing laundry, it's reasonable to request some fair payment for this. This is because you want to avoid a situation where the parent feels like a financial burden on the child. As you'll see below, financial stress can be harmful to relationships.
Now, my moral interpretation aside, I think I can construct some solid, logical reasoning for now charging the parent rent:
- Living with your parents is already stressful, and adding finances to the situation will make it worse. Finances are often touted as the biggest source of stress for couples, and that stress can exist in any family relationship that incorporates money. Avoid finance issue to avoid stress for everyone.
- If you're not already charging someone else for rent and have to displace them, you're not losing money, just space. Since it's not a monetary issue, don't make it one.
- A parent will usually move in with a child when they don't currently have any other options. They may need to acquire a sizable savings in order to get back on their feet. Taking money away from their ability to save could prolong their stay.
- Unless you're already a landlord familiar with the rental housing market, you won't have an idea for what's a fair market rate for a room-renting situation. So, your price may feel arbitrary and cause contention.
- Charging someone for rent may have legal repercussions, depending on your local laws. Specifically, it may mean your parent gets renter’s rights and obligations, and you get landlord rights and obligations. This could complicate matters later if something sours with the relationship, or something with the living situation becomes unbearable.
- Having this additional source of income may affect your taxes, and/or any income-based benefits you receive and/or income-based fees you pay.
- Your parent is coming to you for help. Charging them, even if they're willing to pay, may adversely affect your relationship with them outside of general money-related stress. You may be working against the mindset (verbalized or not) that the parent spent untold thousands of dollars and hours to bring you up, and they're just looking to you for shelter and comfort. You may not agree with such emotions or reasoning, but they can't be discarded as a possible factor in the parent's view of the situation.
So, I would say that the child should morally and logically not charge his parent for rent, in general. However, recouping certain expenses that would not normally be incurred is perfectly reasonable in order to avoid financial stress.
If some sort of repayment for these incidental costs is going to be requested, then the expectations need to be calmly and clearly laid out well in advance. Both parties need to be on the same page regarding the flow of payment here.
I personally wouldn't recommend a direct-billing approach, in the "you owe this much" vein. Instead, I would find other ways where the parent can pitch in with the household needs that has a value that approximates the new costs.
As @Erica suggested in the comments, pitching in for groceries is one good way to accomplish this. Putting gas in your car (if you have one, and they borrow it), is another example.
Non-monetary exchanges, such as doing household work, are also a solution. However, the level of work shouldn't exceed the value of the costs. Again, this is to avoid generating conflict. In this case, you don't want the parent to feel obligated to perform excessive work, feel undervalued, or feel like an indentured servant.
I would add a line of caution regarding forgiving payments or gifting money/shelter. This should only be done with no strings attached. If in the moment you don't want repayment, then that needs to be final. It shouldn't be brought up later, as in "I let you stay for free for [x] months!". It should be considered a gift forever and always.
If the situation changes and you feel you do need to start receiving some money to cover your increased expenses, it should be only for that moment forward. There should be no back-log of expected payments.