To answer your question, as far as I know, you can feed corn (as part of a balanced diet) at any age, provided it's ground fine for chicks or if they're older, into a coarse grind if they've access to age appropriate grit. The easiest way for grit is to free range them and they will find the right grit. If they are penned up, you need chick grit for chicks and a coarser grit for adults.
So true! The only time my flock would find anything even remotely useful as grit on their own while ranging is if I let them venture clear down to the creek back in our woods (which I have recently deemed off limits due to a new neighbor and their dogs) - and even then it's slim pickings to find anything other than clay and mud, bleh!