Some good points above but I don't think it is that important. I don't know when I first see chicks with a broody hen start dust bathing. I've never paid attention. If it is like everything else with chickens, some will start really young and some won't. I keep chicks in the brooder for 5 weeks without providing any place for them to take a dust bath. It does not seem to hurt them.
Are there benefits or potential risks in giving them something to dust bathe in? Yeah, probably both benefits and potential risks.
I see this as another one of those things that affects humans much more than the chickens. If you want to provide a dust bath then provide it as young as you want to. It should not hurt them. If you want to wait, then wait. That should not hurt them either.
I understand you want to do what is right and what is best for them. Most of us do. But I just don't see where it matters when they start.