Normal vision and binocular vision
Visual acuity is developed before age of seven
When we are born, we have all the conditions ready in the visual system. However, the visual acuity is not developed in an infant. The brain need visual stimulation to establish a good visual function and for a normal visual development to occur, both eyes need to be able to have a clear image on the retina. If these conditions are met and the visual cells and brain cells are stimulated enough, visual acuity can be developed normally. All infants see details poorly but in time for school start, most children have developed a normal visual acuity. Some parts of the visual system are still under development up to early teenage years, such as contrast sensitivity.
Normal binocular vision demands eyes that can cooperate
If both eyes have good visual acuity, similar size of the retinal images and is able to focus on the same object at the same time, we have a good starting point to develop binocular vision. But, if some of these conditions are not met, as in strabismus, a sharp image in the central retina is not obtained in one of the eyes. The visual development in the strabismic eye will suffer as well as the binocular function.
It's possible to treat a strabismic child to achieve good visual acuity on both eyes by doing occlusion therapy but it's more difficult to get the eyes to start cooperate and achieve a binocular function if this is not developed normally from start.
Other reasons for an abnormal visual development can be juvenile cataract (the lens in the eye is not transparent), nystagmus (fast, uncontrollable movement of the eye) and refractive errors (often far sightedness). All theses conditions is prohibiting the image to be sharply projected on the retina and the visual development is compromised.