It depends on you loft set up!
If your youngbirds have access to a flight then they will see there surroundings as soon as they are strong enough to get into the flight.
If the front of the loft is closed in then as soon as you wean the youngsters put them in a basket each day and stand it outside to have a good look around.
An old friend of mine used to have a table stood about 20ft infront of his loft with a wire cage fitted on top of it, the youngsters would spend most of the day out in the cage and when it was feed time he would whistle them, taking the one by one and posting them through the traps. After doing this for a few days all he had to do was whistle the birds open the cage and they went straight to the traps and in. This was done for a few days before the birds were allowed to fly out of the loft on there own.