The first checkup can be done at any age if you notice anything unusual to the child (squint, winks a lot, different pupillary size, color ...). Although children under 3 years of age will not be able to know exactly how much they see, we can have an idea. In addition, you can assess motility (strabismus), refraction (glasses), fundus, cataracts, eyelids...
If you do not notice anything unusual it is convenient to do it between 3-4 years, and a second at 6 years. This is because there may be some problem in the visual development of one of the eyes and not being detected by the parents. The child sees well with the other eye, and as it has always been so for him and he sees it normal, he does not say it.
Vision development begins from birth to about 8 years. Any problem, whether refractive (glasses), strabismus (twisting the eyes), opacities in the media (cataracts, drooping eyelid ...), etc., that interfere in the arrival of a clear and quality image to the brain goes to prevent the formation of neural connections necessary for vision, appearing the so-called "lazy eye" or amblyopia, which is the correct term.
To solve the amblyopia, you must first solve the problem that causes it (put on glasses, operate the cataract, drooping eyelid ...), and then carry out penalization measures (patches, etc.) on the healthy eye to balance the connections of one and another in the brain.
The younger, more plasticity (ability to change and adapt) has the brain and is more likely to solve amblyopia. After 8-10 years it is very difficult to recover the amblyopia or "lazy eye".
There are still a significant number of children with undetected amblyopia that could have been solved.
For all this, it is important to remember the need for checkup at 4 and 6 years.