Definition: to start living a quiet life in one place, often demonstrated by buying a house and / or getting married.
(This phrasal verb has more than one meaning)
E.g.1: At the age of 32, he decided to stop travelling and settle down in a quiet town in England.
E.g.2: You shouldn’t still be going to discos and nightclubs at your age! Don’t you think it’s time to settle down?
This phrasal verb cannot be separated.