We usually move our chicks from free ranging to fenced pastures when they're about a month old. The transition isn't for their sakes but for mine --- older chicks have a tendency to scratch up my mulch and peck up my strawberries.

This set of pullets preferred browsing the far edge of our yard beneath the pear trees rather than invading the garden. But at six weeks old, they'd dramatically outgrown their brooder. Off the coop it was to learn grownup perching!