Usually when I start losing space in the parents house or its getting too chaotic to play in which i either have to move someone out or scheme for death
recently though I have quite often used either going to university or getting married as the reason for moving out
or maybe just want to explore different world than where they were born in would do
+ maybe if i ever get hsy I might use that as excuse to send them to copperdale as exchange students n such to make it earlier for some
but yeah idk I mean as long as theres space in house and its not great trouble to play all of them i think its all fine to live with the parents