Hard disagree. I am not a linguist, but did study language acquisition a bit in the context of childhood development and unless the science on the topic has changed dramatically in the last decade, it seems pretty clear that there are physiological differences between child and adult brains that dramatically impacts language learning.

For example, there is a critical age period for being able to distinguish different sounds, something that if not learned during this period may be impossible to ever pick up. This age period is shockingly young; I don't remember exactly but iirc it's less than one year old.

The most well-known example is that in Japanese, R and L are the same letter (their R/L letter sounds like a cross between the two, with a bit of D thrown in). Thus Japanese people have difficulty distinguishing between R and L in English; I personally verified this with a bunch of my Japanese friends (including a number who spoke English very well) and they could not distinguish between "election" and "erection," no matter how clearly I enunciated. However this is far from the only example out there; native English speakers similarly struggle differentiating various sounds in languages from countries like India and China that are clear as day to those speakers. This is not a matter of will or attention or even practice, it's a brain issue.

Given this, I find it highly unlikely that there aren't other elements of language learning that are harder (or even impossible) to properly learn outside the critical window.