What is the best age to start teaching children a second language (English is their first language). Thanks !

When they are like in 6th grade.

well first of all how old is the kid? if they haven't already learned the English language i think it still wouldn't hurt to teach them another language. kids are very smart i learned how to read and write when I was 5. My cousin learned how to speak 3 languages when he was 7 so i guess you could teach them more than one language starting from the talking phase

they say the earlier the better, like when they start saying mommy and daddy .

The earlier the better. Kids at a young age absorb things like learning a second language like sponges.

The earlier the better. When children are born they have all these nuerons that are firing and connecting until about age five. After this point the brain starts to prune the unused nuerons making it harder to develope another language later. If you or your spouse are bilingual it is best for one parent to speak in the primary language and for the other parent to speak in the new language at all times. This way the child is exposed to both equally while the brain is still making connections. This is not to say that you can't learn a new language later in life but it is much easier for a young child to pick this up.

When they are pre-verbal.

The earlier the better. I would say as soon as they start to talk.

We learn language best before age 7. If you can expose your child to another language then, you should.

I learned both German and English from infancy up...I think the earlier the better!