By mixed corn, are you talking about a mix of scratch grains or different varieties of corn?
They CAN eat some grain at any age. But the question is should they?
If a mother hen brought chicks out to forage she would show them what to eat and that would include seeds/grains. However, Chick starter feed (and all other feeds) are already mostly corn, grains and legumes so there is no need to add more of what is already in the feed and will lower the protein and some of the vitamins and minerals in the feed.
If you have a complete chick starter feed, that will give them the best growth and immune system than mixing in more of what are already the primary ingredients in the feed.
Have you read the ingredient list on the feed?