That said, I am still curious about all this and how it particularly relates to or is impacted by dual immersion programs. My son is not bilingual by any means but my wife tries to teach him very very basic Mandarin. The hope is that he will become bilingual once he's in the immersion program. Structurally, the daily courses are taught in 80% Mandarin/20% English and starts at K - there is no TK program for the Mandarin Immersion Program that I'm aware of but I should actually check. That said, I have heard that because the curriculum for the immersion program obviously has less of a focus on English, science, and math, that many parents end up spending more time at home with their kids 'supplementing' (I'm not really sure on what that actually means though but maybe I should ask more about that haha). It sounds like naturally though, kids (regardless of their age) may not be *as* up-to-speed with those other subjects due to the emphasis being on learning the second language. So I wonder if that may make it somewhat of a moot point that we feel our boy is "behind" and are concerned to the point that we would consider holding him back.
I tried to address the immersion thing earlier, but I'll go into a little more depth. My kids are in 2nd and kindergarten at a Spanish immersion school, and they each did two years of Spanish immersion preschool. My Spanish is terrible, my husband's is pretty good, but we predominately speak a dialect from Spain, which makes it a bit more difficult as it's pretty different from the more common Latin American dialects in our area.
One kid has a long-standing speech delay (he's been in speech therapy since 2), the other is pending an ADHD diagnosis, or is just-sub-clinical on attention deficit. Basically, while they're good kids, they're not any sort of idealized student or gifted stars or anything. Our parenting philosophy is much more on the side of encouraging questions as a way of learning than rote memorization, so they've never had flash cards or learning exercises or any of that kind of supplementation at home.
Both kids have learned plenty in English, despite never having formal instruction in it: counting, the English alphabet, animal names/noises, etc. Your standard early childhood stuff.
When we were looking for an elementary school, we toured our local Mandarin immersion, because we had neighbors who went there and loved it. It was a great place, excellent test scores and the kids seemed happy, but it was very worksheet-intensive and we didn't feel it would be a good fit for our kids or our parenting style. There's no "catching up" at all; that school is at the absolute top of the district in terms of English-language testing, although I think a lot of that was that they had class sizes of 15-18 for the first couple of years they were in operation. It's become more popular now, so they're at the average size of 26 for the early years, but no word yet on what the test score impact will be.
Our kids' immersion school doesn't start any English instruction at all until 2nd grade; before that everything is Spanish, and parents are asked not to do any formal English phonics because it can disrupt the Spanish pronunciation. Prior to this year, the 2nd grader wasn't all that interested in independent reading. He could sound out any word in Spanish, but he frequently didn't know what the words meant, so he wasn't comprehending what he was reading except in the simplest books. On the flip side, his English vocab is great, but he couldn't sound out words because he hadn't learned English phonics.
But about a month ago, it was like a switch flipped, and he's reading chapter books in both English and Spanish, and he WANTS to read and go to the library. I think it just took a little bit of phonics and he was able to build on the reading skills he had developed in Spanish and transfer them to English. I'm still concerned about his spelling, which is awful in both English and Spanish, but his teacher said it's age-appropriate. (His to-do list this weekend included "clin rum" and "clin ap stars".)
The kindergartener seems to be on about the same path, although he's trying to read and write earlier than his brother did, probably because that example helps.
Both children are about average in terms of academic achievement overall, but they're at the higher end of their school because it's a Title I school with a lot of low-income and English Language Learner students. Age is definitely far less of a factor in determining achievement than socioeconomic status.