I waited till eight weeks before offering anything other than Start & Grow feed. But I put them outside at 5 1/2 weeks, where they had access to grass, weeds, worms and bugs. So I offered them Chick Grit granite to digest those goodies.
At 8 weeks I gave them scratch grains as a treat once a day, what they consumed in 10/15 minutes.
I would probably wait till 10 weeks before offering scratch grains for my next flock as I think that the complete nutrition in feed is critical for proper development of the egg laying tract.
One of my 5 hens, the last to lay, laid many soft shelled eggs and weird eggs in the beginning and still lays 3/5 a month at nearly 15 months.
Ideally in my opinion, chicks should get nothing but feed for 16 weeks. But that would be like telling children they can't have chocolate covered ice cream till they are 16.
Feed them treats when you think you should, just very limited. GC