I give access to the outdoor world starting at a week old. I'm not sure what the "right" age is but have heard from folks here that taking a little scoop of dirt/weeds/grass and putting it in upside down so they can pick through the roots is beneficial to chicks housed indoors. They'll probably avoid it for a few days yet but a mama hen takes them off the nest around 3 days. I only wait because I need them to return to the shelter and locking them in a few days enforces that they eat, drink, and sleep there.
Day three. Seriously, so long as they have chick grit to digest it. What do you think Mama would feed them, breast milk?
Pull up a clump or two and leave the dirt clog attached, this will not only acclimate them to the local microbes but will help them build up a good colony of intestinal bacteria ("probiotics") as well.
As soon as they are eating well, filling their crops with their crumble, I start them on a clump of sod, or a dish of soil from my untreated yard. I am also a fan of fermented feed. Early is good for exposure to soil as it takes advantage of the 2 week window of opportunity when they have the most antibodies inherited from their mother. Be sure they have access to grit: most soil has acceptable stones they can use. And leave the blades of grass attached to the roots.