LORELAI: It’s just that – you know, it’s about the freedom. I mean if I had access to all that money as a kid I would have left the house so fast.
SOOKIE: Faster than seventeen?
In fact we learn in the next season that Lorelai was actually eighteen when she left home in 1986. Originally the show seems to have decided it would happen in 1985, when she was seventeen and Rory almost one.
The Annotated Gilmore Girls 