I get your point about time availability. This however doesn't explain why it's common for young adults to learn faster than older adults. Often young adults a

I get your point about time availability. This however doesn't explain why it's common for young adults to learn faster than older adults. Often young adults are even more time constrained than older generations (new family, young kids, possibly less stable job situation, etc.) and yet older adults are usually the ones who have a harder time learning a language (as a "serial" migrant I have experienced this personally). So I don't think that time is the only factor.