My parents quite gleefully kicked us all out immediately after high school graduation. They gave us plenty of warning and "gleeful" is the right word. Being very "sink or swim" in their thinking, they provided absolutely nothing in terms of help or support. It wasn't a good thing for any of us.
Me: I worked at a summer camp, so I literally left the day after high school graduation. Then I moved into a dorm, then a series of apartments -- always with roommates; at the worst, five of us in two bedrooms. I married at 24, and we bought a house that same week.
My parents' exception: One of my brothers was assigned to go to Boot Camp some 4-5 months after graduation, and they reluctantly agreed that it wasn't reasonable for him to take on a lease, etc. when he was leaving so soon. He stayed in his high school job /lived at home, but they charged him rent and constantly reminded him that as an adult, he had no business doing what he was doing.
Our oldest: Graduated from college, came home for about six months ... then married and moved into an apartment. When she was in a college apartment we co-signed, and I paid the deposit /told her she could keep that for a deposit on her next apartment -- she did. We paid for her housing during school, and when she moved into the apartment we provided her with some hand-me-down furniture. Now she and her husband have a nice house of their own.
Our youngest: Will graduate in May. She's also in a college apartment, which we are paying. Again, we helped with hand-me-down furniture and told her that she can keep the deposit for a next apartment. I anticipate she'll come home /stay longer than her sister did. Two reasons: She isn't getting married soon like her sister was, and she's going into a career that isn't known for paying well, so she's made two decisions: she plans to live at home (which is FINE with us and costs us next to nothing) until she has $200,000 to invest towards retirement ... AND until she has money to buy a house to rent for income. I think she's making good choices ... staying at home until she's solidly settled. A good financial start will allow her to build financial wealth while working in a job that pays only a modest wage.
My oldest DD will be 23 in 11 days. The deal was she can stay here rent free as long as she was working or going to school. She does both.
I'm completely in agreement: My kids can stay here in my house as long as they want. I live in the house anyway, and the bit of extra electricity and hot water they use doesn't signify. BUT I do expect them to be in school or working. I'm all about helping them get a good leg up on life ... but I'm not supporting laziness.
I was talking to my oldest friend yesterday. Her youngest daughter is 17 and the friend said she thinks the daughter will move out before the end of the school year (she turns 18 in November). Her oldest daughter moved out with her step-mother at 15 and then boyfriend well before 18.
I've been teaching high school seniors for years, and it seems that every year I have a senior girl (always a girl) living with an older boyfriend.
I have yet to see a high schooler in such a situation who is thriving. They aren't ready.