Its good to follow up early for a number of reasons.
DH has an eye condition on his right eye. Im not sure what it is, but he says even with glasses he cant see that well with that eye.
Anyway, it wasnt picked up until he was 4, and they tried a therapy to fix it which worked while his "good" eye was covered, but as soon as they uncovered it the vision in his right eye regressed.

The specialists told his mum that if they had been able to treat him earlier, the eyesight may have been permanently corrected by the treatment.

Im not sure exactly what they did but it involved covering one eye and looking at those black and white spiral patterns ona spinning disc, so I guess some sort of occupational therapy.