As soon as they start eating anything other than their chick feed. If they are being raised by a hen, it's safer to give them access to grit from day 1 because she'll teach them what's food and what isn't and how to eat properly. If you are raising them in a brooder though, they have no guidance and will try to eat anything. To them, a grain of grit will be just as "food" as a grain of feed, so they can eat too much and get themselves clogged (or even die). So for brooder-raised chicks it's better to wait a week or so, until they have learned what food is and are reliably eating their crumble (as opposed to bedding etc.), so they won't gorge on the grit when they first see it.