What breed, what are you feeding them? You can put it out any time, but they most likely won't show interest until the first eggs are ready to appear. If they are on layer, they really don't need it, but I like to keep some out anyways.
I would offer at 16 weeks in a separate container.
I waited till I got a few soft shell eggs with my current flock. Didn't think I needed it, feeding a Layers feed.
But I was too generous with the Scratch Grains in the beginning.
I am currently feeding a Non-Medicated Start and Grow to my three 2 year olds with Oyster Shells hanging at the Pop door, near a water source.
.GC
Put it out when you think of it, by the time some are laying eggs. By four months of age, anyway. There's no magic time, just have it there before it's a problem.
Mary