Mine had some at about three days old. They liked it and ate it well with no problems. I wasn't home then and didn't have a feeder that I could clean easily so I went back to dry feed for a week or two.
Then they ate both/either for a couple of weeks. The brand name crumble disintegrated; I like the ground feed from the mill much better for soaking/fermenting. And almost exclusively FF once the small bag of crumble was gone - when they were maybe 4 weeks old.
If I get chicks again, I'll most likely do only soaked/FF or dry for a couple of days then only soaked/FF.
My chicks have access to FF at around 2 weeks (when they start getting run access they also get access to whatever food bowls are lying around for adults) but usually won't start eating it right away, so I'd say they start eating at around 3 weeks or so. As a precaution I offer fermented starter to all the birds, so it doesn't matter how early on the chicks start eating it.
In the same vein, layer pellets are for active layers, so if you want to offer FF to chicks, best to ferment a starter or all flock type feed. Once they're laying or close to it you can switch to layer if you prefer.