My Cantrell kids moved out when three of them were Teens, Early Graduates of HS and the eldest was a YA. They all went to uni together. All but the sister now live with their Rock Star brother and his young family. (That's okay, he's got a bit of mansion "cottage" in HoB.) When I'm totally ready to let the eldest and the General Practioner live on their own, I'll simply have them move out. I may or may not build their homes for them. I already use the Jessica Fletcher house for the doctor, at least I did in the Main save prior to this one. I'll just have to wait and see.
https://i.imgur.com/8QRnDnJ.jpg