At what age do baby chicks began to roost?
It really depends on the individual personality of the bird as well as how quickly they develop. Birds that feather out quickly may begin to roost early (if they have a roost to use), since the grown-in wing feathers allow them to fly up to the roost easily. Some may try roosting almost immediately, while others may take two or three weeks.
Most people don't put roosts in their brooders, in which case it may take a few weeks for older birds to figure out "roosting" when they are moved to the big girl coop. However, roosting behavior depends on the breed, too. For instance, silkies sometimes don't like to roost even when they are adults. Flocks of silkies sometimes "roost" in a fluffy pile on the floor.