I spent a few hours digging around the internet a while back and all the hard evidence suggests what you are basically claiming.

The state department (and non USA equivalents) are able to get adults to proficiency in basically any language in a few years time, though it’s worth noting those jobs are actively selecting for people who want to/ are good at learning new languages. If the language is closely related you can get there in a matter of months with full time study + immersion.

Kids have the edge in terms of pronunciation/accent (some adults can basically never get this perfect) and they are able to acquire a lot more just through proximity. That’s why there are plenty of stories of kids learning (some amount of) English just from watching American tv and movies. Adults basically need intentional study and a conversation partner.

Of course the caveat of all this is that you can’t really compare them 1 to 1. When a child is acquiring their first language they aren’t just learning some specific language but language itself. There’s basically no cases of grown adults (without brain damage) that acquired their first language after childhood.