Updated Jul 28, 2019
It depends on the individual circumstances and attitude.
When I was 18, I moved from my mother to my father, specifically to be in an area with better job prospects. And was living on the lounge floor of his cheap apartment.
His rent stayed the same when I moved in. I used very little electricity, and ate modestly. I had arrived with only a carload of physical possessions, and was quiet and low-impact. I cleaned up after myself, etc.
I went out and got a physically demanding factory job for minimum wage, and started paying for the petrol for a long work commute. Aside from shelter/electricity/food, I paid for anything else I wanted/needed. There were basically zero luxuries. Most of my income went into a savings account.
I knew that, the “free” roof over my head was very temporary. And was focused on working and saving to get started with total independence.
And nobody had ever had to tell me to do this. Nobody ever had to tell me that, adult responsibility was approaching. It was just patently obvious to me.
After just a few months, when I was still eighteen, my father and I parted ways. The demarcation point was his relocation to another city, and my voluntary choice to move to a different other city (since adults don’t have to get dragged around every time a parent moves). And I started completely, 100% supporting myself. I think he sent me a cheque for $50 for Christmas, on one occasion when I was about 20, and nothing else ever again.
It never occurred to me to try to live with either parent past age eighteen. I haven’t even been located anywhere near either of them in thirty years. In that time, I have been all the way down to homeless, and it was still never a possibility to go crawling back to either of them. (Their bad attitudes about how I choose to live my adult life also made that idea unviable from a very early point).
And that wasn’t nearly as hard as some people have it.
One of the coolest human beings I’ve ever known, started working, paying rent, living independently of parents, at age fifteen. And she never felt at all sorry for herself over it, and mainly described it as a challenge that she rose up to deal with, and to then go much further in life.
On the other hand, there seem to be plenty of pseudo-”adults” who feel zero sense of responsibility for supporting themselves. They feel entitled to act as if they were totally independent and in-control, while living rent-free under someone else’s roof. On and on and on, all the way to expecting a permanent, lifetime free ride.
Some of these pseudo-adults are well past the age of eighteen. And some are willfully ignorant of how much things like shelter, food, transportation, etc, cost. Some are even in denial that, obtaining money requires an employed person to expend great time and effort, whether s/he likes it or not. They think resources just magically appear.
Some may work, but then see the “free” shelter, food, etc, as enabling them to blow all of their own earnings on luxuries (video games, junk food, partying, etc, etc).
I have encountered multiple individuals who thought that, they were going to find someone who was NOT their parent or their spouse/partner, who would serve as permanent, limitless host to an openly abusive, hateful little parasite. They called this concept “friendship”.
I had to pay rent when I was nineteen. Because, at that age, I was already living in my own (modest, one-room) apartment. In a low-rent, high-crime neighbourhood (thousands of kilometres away from either parent). Fully self-supporting, working long hours at a regular, demanding, grownup job.
If your young-adult offspring is living with you, then it needs to be his/her priority to either work or pursue higher education.
If working, s/he should be exercising restraint by putting some money into savings. And also accepting responsibility for at least some expenses, such as petrol, lunch food, etc.
On one hand, charging rent could emphasise the fact that basic needs cost money, and that they will have to pay eventually. It can also make staying with parents less appealing, and prompting them towards independence.
On the other hand, if they are already working towards independence, charging rent could backfire, by making it harder and more time-consuming to save up to move out.
I would be inclined to only suggest a “pay rent or get out” ultimatum if the individual is abusing the “free” shelter situation for an extended period of time.
At that point, if they don’t like the situation, they can go out and pay rent to a real-world landlord, who may charge far more, provide far more modest conditions, and be far less tolerant than Mommy and Daddy.