The only stupid question is the one not asked.
Chickens rely on their gizzard to grind up certain foods so they can digest it. If all they eat is chick starter, they don't need grit, but for almost everything else, they do.
Grit is sand or small rocks that the chicken eats that goes to the gizzard to help grind up the food. The gizzard cannot gring food without it. Most of the grit sold is made from granite since it is very hard and lasts longer. If they eat sand or small rocks made of another substance, like silica or limestone, the grit will be ground up sooner and sooner passed on through their digestive system. They will then need to eat more grit. If they have access to the natural ground they are probably getting sufficient grit.
Some people get oyster shell confused with grit. They are not the same thing. Actually, chickens need grit to use oyster shell. Oyster shell supplies extra calcium, needed by the hens for their egg shells. If your natural stone is limestone, they are probably getting plenty of calcium from that. They also get calcium naturally from other things they eat, such as hard shelled bugs. Extra calcium should not be given to chickens before they start to lay or are of an age to lay, say around 20 weeks.
When a chicken needs grit is not an age thing. It depends on what they are eating. Many people successfully do not offer their chicks grit as long as they are eating starter and are not giving them any treats. I personally like to offer grit on day 3, just because I don't know when they will ingest something they need grit to help pass through their system, like when a hard-shelled bug wanders into their territory.
Hope this helps.