I think whenever you're able to support yourself.
I've lived on my own and with my parents since 18.
My friends and I rented a house when we were 18 for a year, I moved back with my parents for about 2 months, then got my own apartment with my best friend, then went back to my parents for a while, then got another apartment by myself, then my parents wanted both my sister and I to move back in because they were bored and it saved us money, so we've both been living with them for about a year now.