How old should a child be to correct crossing eyes.?

Earlier the better. Newborns often cross their eyes and may not move their eyes smoothly together and this is normal. However, after about 3 months of age their eyes should be moving together and not crossing. If they do cross see your pediatrician who may recommend an evaluation with an eye doctor. Treatment, which may include an eye patch, glasses or surgery can be started even as young as 4-6 months.
Do not wait. Young infant have a hard time focusing their eyes.Babies with a flattened nasal bridge could at times appear to have a "lazy eye".However when the problem is persistant, seek professional help asap.Your pediatrician is a good starting point, if he or she agrees with your concerns, a visit to a pediatric ophthalmologist is a must.Untreated, it could lead to permanent visual damage.
After 6 months asap. Babies can have a small amount of "cross of the eyes" which should really go away by 6 months. If it is severe or doesn't go away by 6 months the first step is to go to a pediatric eye doctor who specializes in this condition (strabismus). Speak to your pediatrician about any concerns you have. Treatment can include patching, glasses, eye drops and surgery.