Foreign language education in the United States has been essentially exiled to the depths of high school for one reason; lawmakers, and parents alike, feel that there are more important things to teach than language to those young, developing minds.
Why waste precious school time teaching French to an American student when more practical things like Math, Reading, Social Studies, and English are needed to excel in this country. Where are we going to find early childhood education teachers that also speak multiple languages? Are we going to have to pay them more?
It's a logical start-with-the-basics and then add-the-superfluous-later type of mentality. It is also, however, flawed.
Children have an uncanny ability to learn a lot of things. Part of what learning does is helps you learn how to learn more. Learning, and processing information, helps strengthen the brain and train it to learn additional things later in life. The same task can be taught to a 30-year-old much easier than it can be taught to a toddler. Our brains develop, grow, and strengthen; but so does our ability to learn.
Interestingly enough, learning a second language does exactly that. It promotes brain growth in the area responsible for spatial navigation and learning new material. If you rephrase the question then, it would be, "How early should we start promoting brain growth and enhance learning in our children?" When put that way, it sounds fairly ridiculous.
One cumbersome question still remains: How can we do this without affecting learning other things. Learning languages takes quite a bit of time which is in short supply these days in the educational environment. The best option is to provide an immersion environment for the children.
Learning a language is a beautiful thing because it doesn't have to specifically be taught to children. They can learn a language through consistent, sufficient, and quality language exposure at the same time as they are learning something else. For example, if a teacher teaches a math class through the medium of a different language, the child will learn math and the new language at the same time. This is essentially squeezing a huge amount of free time throughout the day.
The pitfalls of this strategy are well-documented. Finding qualified teachers who speak another language may be a challenge. Coordinating the whole thing also creates quite the headache as well as just the massive amount of change that would have to take place. Culturally, it's not to a point where it would be acceptable in the mainstream yet; however I feel like we are moving in that direction.
This option seems like a very viable way to learn a new language. If children don't have parents who speak more than one language and can pass it onto them, I don't see why parents shouldn't have the option to take advantage of that great gift in the form of formal education. Many private schools, as well as schools for the wealthy, are already and have been doing this for the simple reason that it is effective.