Convergent squint is most common in infants and young children, at which time it is particularly dangerous to sight because vision in the squinting eye is suppressed causing amblyopia, which, if untreated at the time of its development, becomes permanent.
This loss of high definition vision is greater the earlier the squint occurs and the longer it is left untreated. It is totally reversible if the squint is treated early.
Even if the vision is restored to a squinting eye by occlusion treatment - i.e. putting a patch over the better eye - there may remain a loss of binocular vision. This too is dependent upon the stage at which treatment is begun, so it is essential to refer a suspected squint as soon as it is noticed.