I think there's high variation in adults' ability, low in children's ability.

A million or more Jews came to Israel from the USSR in 1990-2 (my family among them). Virtually none of them knew any Hebrew (studying or teaching it had been illegal in the USSR), or any other language spoken in Israel (English, Arabic). The immigrants had to learn Hebrew to be able to get jobs, attend school/university/retraining, and interact with local authorities. Local Russian-speaking communities did not yet exist on the requisite scale to take in more than a few newcomers. There was enormous pressure, and all the motivation you could want.

My family was among these immigrants (I was 6 years old at the time), and I've met hundreds or thousands of others. There is a very strong negative correlation of age and fluency in Hebrew, even today, after 18 years of immersion. People who immigrated at age 10-15 or younger are fluent speakers, have little to no accent, and it is very remarkable to meet an exception. Among people who immigrated as adults, most are fluent but make grammatical, pronunciation or spelling mistakes, or have vocabulary problems. Among people who immigrated at age ~~ 50-60 or older, many (I estimate at least 20-30%) never progressed beyond rudimentary skills necessary for e.g. shopping, cannot read or write fluently.

There is of course strong correlation between environment post immigration (school, work, non-Russian-speaking workplaces) and ability, but I've met enough such people to know that the causation goes both ways.