Would you let your son or daughter older then 18 still stay in your house or would you start mentioning apartments and other spots?

I would let them stay. In large part because it is not 1970 anymore. Rent is absolutely insane. What I paid 10 years ago for a 3 bedroom townhouse would now barely get you a 1 bedroom apartment in a neighbourhood that is not dangerous. For context that is over $1000 a month. Something someone just starting out cannot realistically afford anyway.

Even those who live on their own are often being subsidized by their parents anyway.

Basing "leaving the nest" on social norms from the 1950s no longer makes sense.