Wait until you get your first egg. Kinda bragging here but i jiust got back from buying my first bag of layer feed cause i got my first egg this morning. I would suggest using crumbles or pellets. The mash is very fine and they can waste it. I'm using crumbles. They are the same thing just in different forms.
The rationale is that layer feed, whether in crumble, pellet or mash, has more calcium than young birds' kidneys can handle. But laying birds need the extra calcium because they're manufacturing egg shells. And I guess the adult roosters' kidneys can now handle the extra calcium.
I start my chicks at 18-20 weeks whether they've laid their first egg or not. Layer feed is cheaper than starter, and I'm a cheapskate.
I give them pellets because they waste less when they spill it on the ground, pellets are easier for them to find & eat.