Have you tried offering oyster shell on the side? It doesn't always work but it usually does. Was that hen previously laying hard shelled eggs before you switched to chick feed? I never feed layer. My entire flock gets a low calcium feed with oyster shell on the side.
There is no specific age where they are too young or conversely, old enough. There are just too many factors involved. Do they forage for most of their food? Do they get low or high calcium treats. It's not how much calcium is in one bite, it's how many total grams of calcium do they eat in a day, and even then it's more of an average of how much over a period of days.
Then you get different tolerances by the individual. In those studies where they show that excess calcium can harm a growing chick (and it can) not every chick is affected the same way. More die than normal, some are permanently damaged inside, but some are not harmed. I have no idea which of those your chick would be. In those studies they only eat Layer so that's a variable if ours eat something else.
The studies that have been performed are generally in broilers that are butchered before 3 months of age or on the hybrid layers where the commercial operations can determine when they will start to lay. Those studies do not apply to our chickens, we don't keep them the same way. Wile I am convinced that you should not feed your growing chicks Layer if that is all they are eating, it is less of a concern of half their food is from low calcium sources. Still I consider it good practice to not offer them Layer until they are laying. Until them see if offering oyster shell on the side works.
