Recent studies indicate that the best period to start the learning of L2 is preschool age (first three to four years). Can a foreign language be introduced simultaneously with the L2 at this preschool age or it should come later?
I'd be interested to know which empirical studies indicate that this is the best period.
The broad consensus for early foreign language learning is it does not harm own language acquisition (Johnstone, 2009).
"Should" is a broad policy question that we can only answer in light of the specific goals and available resources. As researchers, we can really only answer the question "Does X lead to an outcome of Y" or "During the process of A, participants experienced XYZ" or some combination or reiteration of the above.
Johnstone, R. (2009). Review on research on language teaching, learning and policy
published in 2007. Language Teaching, 42, 287–315.
Just to be clear, by "foreign language" do you mean a third language to be introduced with L2 at pre-school or later?
In my own experience with bilingualism in my three children, two languages (English/Spanish) were introduced simultaneously from infancy. To me, from birth would be the optimum time to introduce two languages. (Actually, the third child was introduced to two languages in the womb; she is the most proficient of the three children.) Of course, this would mean that one or both parents would be bilingual. Where this is not the case, then I agree that pre-school would be the best time for L1 and L2. As far as L3, my personal opinion with no data to substantiate the assertion, I would delay that until middle grades (6-8) or secondary school (9-12).
I found L.1 more proactive with preschoolers than L.2; should L.3 be introduced is research-able. Can we attempt this for an empirical conclusion? Am ready to team up.
There are lots of examples in the developing countries including Bangladesh, India, Pakistan and other South Asian countries which introduce English and children are learning it without any difficultly. I think method of teaching and teaching material is important.