You can offer them a dust bath at any age, it's a natural thing for a chicken to do so there will be no harm in letting them have dust baths. Dust baths also help kill mites and other external parasites so don't restrict them from it.
It is so much fun to watch the babies enjoy a dust bath. Mine were a few days old before we had their outside brooder box built and they loved it when we put them in it. We put it where they have access to a little bit of grass and sand to dust in.
My chicks started using the dust bath within days of first leaving the coop (about a week old). Mama showed them how and now they do it on their own. She also tried to get them to sleep on the roost at about three weeks. It's 2' off the coop floor and they kept falling off so they went back into the nest box. It's getting cod here so that is probably for the best.
I bring mine inside in the evening and take them back out in the morning. It is still hot here during the day and in the 70's at night. I have been doing this for about 5 days and I think they are already getting used to it. They would jump out of the box I used at first but now they get in like they know what time it is. They love going outside.