Keep in mind that if the baby chicks are raised by mother hen, she will get them to eat anything good that she eats, right from the beginning. There is never an age that they are too young for free ranging or eating some specific food. They are hardy little birds. The big thing is to make sure they have what they need to properly digest what they eat, and that their diet is nutritious enough for them. They grow rapidly and need a lot of nutrition, so if they eat mostly treats, they may not get the balance of vitamins and minerals they need. When eating anything other than their finely milled starter, they will need grit in order to digest it properly.
That said, the bulk of their diet should be starter, not treats, the same way an adult chicken's diet needs to be mostly her regular layer pellet or crumble rather than scratch. Whether baby chicks can eat scratch may depend slightly on the size of the pieces--chick starter has finely ground corn, grains, etc. already. There is nothing different about feeding them corn separately, except in the size of the pieces and in the amount of corn they eat. Corn does not have many vitamins or much protein, so it is found in a very small proportion of their balanced chick starter. Larger pieces of dent corn will be harder for small chicks to manage and digest properly. Use your common sense when deciding which treats to offer, and err on the side of caution, for sure. The last thing you want is to give your chicks digestive issues such as crop binding.
Scratch is just not a complete diet, and will be bad for your chickens, the same way feeding your kids a strict diet of potato chips and cheese curls would be. The BEST diet for you chickens is a nutritionally balanced feed, along with natural forage they find themselves in the right quantities. Chicks are just baby chickens, and have the same need, if not more so, of a balanced diet.
When I was treating one of my mother hens with sunflower seeds, she engaged her week old chicks to eat some of her treats. (It was endearing--she wouldn't eat any until they ate some, and she was so excited about it, and scratched around so earnestly that she sent a couple of them flying...!) They were fine, because they had plenty of grit, and the seeds were small and few. I trust mother hen. However, she trusts you, too. Your chickens have been "trained" to understand that what you give them is good to eat. If it is not so good to eat, they may actually have consumed it before they realize something is wrong.
Many people make the mistake of giving their hens a "healthy" treat of long strands of cut grass. While grass is certainly good for chickens they don't naturally eat it in long strands, which is the way many people offer it: long lawn clippings. In the natural order of things, chickens nip off little pieces, which are easy to digest. Long cut strands can be gobbled up fast... then they may wrap around each other and clog their digestive system. Your chickens trust you; so be sure to offer them only what is in reasonably sized pieces. ALWAYS make sure that your chickens have plenty of appropriately sized grit. And remember, oyster shells are for calcium, not for digestion--it not the same thing as grit at all!