I think that the best time for learning English (or some other foreign language) is from child`s birth, about from 3 or 4 weeks. Of course it is some kind of passive learning.
I would read to my baby some fairy tales in very simple English or talk to it in English. When it starts rather well speak I would teach it some very simple and common words and later some sentences consisting of two, three or four words.

Nowadays, there are some garten nurseries where are children taught English or German language. I think that is the best way of learning foreign languages for children. They learn some base knowledge of the foreign language which can extend in elementary school.
According to me, learning in the early age is the easiest and the best. If we take some bilingual family where mother speaks German and father English, is absolutely natural for a child to imitate and later speak both languages. The child do not learn but it aquires.
Naturally, everything depends on the child. Not only its interest in learning is important but also its psychics or talent. As we know there are some children talented, they do not have problem to understand tenses or remember vocabulary. Some children are not gifted for learning foreign language. They can not remember words, do not understand tenses or grammar, and sometimes needs more time to understand.

Another reason why to learn foreign language in the early age is that later for example at the elementary school there will be another duties related to school. So there will not be so much time to attend learning foreign language.
I think that if the child is learning foreign language at home it is not so exposed to stress to be absolutely perfect than in a school where is either oral or written examined.

I have a cousin who attend 4th class at the elementary school. He has been learning German from the garten nursery a now he is learning English at the elementary school. He knows words and he is pretty good at it. It helps him later to improve his knowledge about the foreign language and use it in his future life.
In the present the knowledge of foreign language, especially English is very important. That is the main reason why to learn foreign languages. The more languages you know the better.