They can be on dirt from day 1. IMO it's more beneficial for them to have early exposure vs waiting, as anecdotal evidence seems to lean towards early soil exposure helping with resistance to coccidiosis.
that ones practically ready to start laying lol, yeah, im sure it will appreciate a dust bath and some sun ... i usually wait till theyre over 2 weeks, and if its warm enough i'll put them in the wire pen, grind the bottom into a sandy spot and let them play and get some sun ..