One of mine lives at home and works full time. He has more disposable income than I do. He pays a modest contribution to the running and cost of the house, which is about half what a room in a shared house locally would cost.
Why would I not show to a grown adult that living costs, and that having him in the house means more food, more gas and electric, a faster broadband than I would pay for, increased council tax (that alone is extra £52 a month as I lose the single occupant discount...), he is on my car insurance (+£40 a month more) to save him running his own car and more.
He currently saves £1k+ a month towards a house deposit, and next year he will be able to buy and move out at age 23 - hopefully understanding what a house costs to own and run.
His brother is a student. I give him money each month, even when he comes back at holidays and undertakes work at local hotel full time. As a student, he needs that support, and cannot work enough hours himself without significant debt or to the detriment of his studies. As soon as he graduates, he has one month to a) get a job and b) pay me what his brother does, if he moves back in.