So how do you know when the youngsters are ready to be let out? For the most part they will let you know. Piling up against the exit looking forlornly through the grill like it's a jail.
Just like the light Sussex Growers you see in this picture clamouring to be out in the sunshine and on the grass, it is not always the best idea to let them out unsupervised. Smaller birds or young bantams may become prey to cats, birds of prey and even crows and rooks amongst others.
Young birds require considerable care as their feathers might not be well equipped to cope. If they have been raised exclusively indoors they will need time to acclimatise before they are left to roam permanently. Try to avoid letting them out in cold, windy or wet weather
Long cold nights will mean the growth rate of young fowl slows considerably and if you are far north like I am (North Yorkshire) where the nights are long the birds may not be able to eat enough to thrive or even survive so you may want to consider some added light in the morning. Always add supplemental light in the mornings to get them off their perch early rather than trying to keep them up late and risking leaving them on the floor in pitch dark.
Early in the year can be problematic as well, especially if the year has had weather like 2015, cold and wet. Try increasing the calorie count of the food you give them. Add a few sunflower seeds or a handful of suet pellets.