Do you ever plan to kick your kids out at a certain age or charge them rent?
Provided that my future kid is a productive member of society (is working and/ or in school) and not violent or a drug addict or something, as long as I can afford to do so and have the space, they will have a place to stay with me rent free.
If I need them to help with expenses, then I will ask that they help. And if I can't afford to have them stick around, then I won't. But if I can, then that is a big privilege and I will let them take advantage of it.
Also if they deliberately aren't working and are just living a life of leisure then I won't let them stay either. And if they eternally are working minimum wage jobs without going to school or training then I will push them to make more money so that they can move out.
But if they are working or studying, saving up, paying off debts, being responsible, etc. I will always let them live with me. Also if they have some medical issues then they can also stay with me as well.
Also once they reach a certain age, like 30, I will encourage them to move out. I am living with my parents but my goal is to buy a condo at age 30.
I will encourage my kids to save up and reach a down payment for a home or condo at around age 30.
Thoughts?
I think kicking your kid out no matter what at an arbitrary age like 18 is harmful. I mean you're a teenager one day and you parents make you homeless the next. How is that okay? Especially with the job market and housing situation being what it is these days. It's the perfect recipe for disaster because it forces them to move in with sketchy friends or partners or into other unsafe living conditions. Who wants that for their child? Children mature differently. One sibling may be ready to face the world alone at that age, while the other will need a little more time.
As long as they are functioning members of society, contributing to chores and the household (including expenses if they earn money), doing whatever they are supposed to do at a reasonable speed (go to college, school, work) and being respectful and your living situation allows it (aka you aren't 4 people in a one bedroom apartment), I see no problem with them staying a little longer. It's a unique situation in every family and with every child. The absolute majority of kids will want to move out for college or when they find their first job and live an independent life.
EDIT: And in my personal experience the only parents who kicked their kids out at 18 were toxic and shitty parents in general. In most cases they were disgusting "I need you gone so my new boyfriend/girlfriend can move in" cases.