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.