Mine stay on starter until they are laying or are ready for the freezer.
They have access to free range once they are around 3 months but get plenty of other things before that time, too and often start going outside in little pens on the ground during the day once they are 6 weeks old. My oldest birds are 8 years old so I must be doing something right. Once they get close to laying they will start eating the oyster shell that is left out for the free range laying flock and they are always nibbling at the laying mash in the big girls' coop.
You have a long way to go before they are ready to lay. Why deprive them of all the nutrients they are getting in starter?