Sorry if that came across the wrong way, but I was just trying to use your "15-year-old level" as a discussion point, there was nothing ad hominem intended. On the contrary, your posts have been very informative for someone like myself with young children, and you're fairly clear on where you stand.
I've read that the average reading level for Americans is estimated at Grade 9 or 10 level... which suggests that 15 is basically "native" level already. Going higher than that, ie. achieving some sort of higher-level "professional competence" in a language, even for monolinguals, becomes a matter of education, exposure, and experience, and I see nothing to suggest it can't be achieved in more than one language. Are there challenges to achieving "professional level" competency is more than one language? Sure. Is it easy? Probably not. But what I take issue with is people who suggest that merely trying to do so will ultimately stunt kids and leave them competent in neither. And I've noticed that people who say so usually did not grow up bi/trilingual. And international schools have below-average kids just like anywhere else, so there will always be anecdotal examples of kids who couldn't pull it off... but it's just that, anecdotal.
I am happy to learn that there’s no misunderstanding. I appreciate that there are people suspicious of bilingualism, but they tend to be American and on a certain end of a politically extreme. I am neither. Actually, I have done a lot of research into bilingualism and have published extensively on the subject, although not recently.
I also am informed by my personal experience as a Japanese learner. My reality is that I have been stuck for the past 20 years between N1 and N2. I have written dozens of academic publications but never one in Japanese. Presented many times but only once in Japanese and even then only as a co-presenter. I rarely read in Japanese and can only remember reading one Japanese book from cover to cover. I have lived here half of my life but I remain a gaijin. It’s simply not my language, culture or values. But I do know that when I return to my home country, I can comprehend everything I see, hear and read. (Well, maybe not the latest teenage slang.) People there get my sarcasm and jokes. My fear was bringing up a child who has no culture in which she is truly comfortable. She would be a gaijin in both (or all) cultures, without any home country.
I wanted bilingual kids but even more I wanted my kids to be happy, healthy and successful. That’s why I got involved in bilingualism. And my children’s and personal experience shows that it’s not easy or straightforward either. That message needs to be out there to balance all those overwhelmingly positive articles I see about the benefits of bilingual brains. I have never read any research about those brains being happier or more content. That’s what is really important and needs to be investigated. I happily pass that baton onto you younger guys.