Hey there! The earlier the better! Some authors talk about something called "Lateralisation"
This is basically how our brains change in the early stages of our infancy. Some believe that before the age of 5 if a language is learned the person may acquire a true bilingual status. This is a little controversial mainly because it wouldn't be encouraging for adults to learn a new language and it also poses the question on what a real bilingual person really is.
Based on what I've seen on little children learning abilities I'd say again: the earlier, the better.
Hope that makes sense
Very good question!