Possible confounding factors:
old people have higher discount rates (‘if I’m going to die next year, why bother?’)
less time to benefit from learning period (years, as opposed to many decades like a kid)
old people have less gain (both from #1 & #2 and because they have support structures which speak their native tongue—their relatives and friends)
old people are just stubborn and dislike the new country and its language (did the old people want to leave? Unlikely).
Possible confounding factors:
old people have higher discount rates (‘if I’m going to die next year, why bother?’)
less time to benefit from learning period (years, as opposed to many decades like a kid)
old people have less gain (both from #1 & #2 and because they have support structures which speak their native tongue—their relatives and friends)
old people are just stubborn and dislike the new country and its language (did the old people want to leave? Unlikely).