healthy human brains are language sponges (my term) when a person is young but changes occur in the brain at about the time of puberty that change that dramatically.
I went to a school that offered a once-a-week Spanish class. I started in 3rd grade. While I can’t claim to be fluent in Spanish, the Spanish I learned then I know as well as English—I don’t have to think about it, I just know it. Languages I’ve learned since always require me to think about what I’m trying to say, how to conjugate verbs, etc.
Offering foreign languages could and should start in Kindergarten—maybe even a couple and continue through grade school/jr. high.
My observation of people IRL is that that is a prime truth. A family friend has a six year old that surprised the hell out of us at dinner one night. She insisted that she order the food for us at the local Chinese restaurant. So we humored her. She proceeded to order the meal in Mandarin. Her mother was as shocked as the rest of us. The kid had learned it during their two hour per day commute off of an ipad. She told me over Thanksgiving that she is working on Spanish. I have known adults that can’t even order the basics in Spanish, let alone Mandarin.
That's why we don't teach foreign languages until high school!
(The point of the American education system is to give children a life long loathing of learning.)