If a child was born with strabismus of the left eye, when is the best time to correct it?

Evaluate quickly. If the left eye is consistent turned out of alignment (usually inwards in this age group), it needs to be evaluated to rule out tumors, cataract, glaucoma and other conditions which might need immediate attention. This is imperative since you say it was present at birth. Other conditions such as ordinary misalignment or refractive issues usually are not at birth and are fixed later.