Any apparent causality of age of acquisition seems to be a proxy of hours of exposure. It may well be that it is easier for young people to rack up a lot of exposure to a second language, but not much evidence that age plays much of a factor for people of different ages who had the same degree of exposure.
If anything, the evidence is that it isn't true, see https://journals.plos.org/plosone/article?id=10.1371/journal...
Any apparent causality of age of acquisition seems to be a proxy of hours of exposure. It may well be that it is easier for young people to rack up a lot of exposure to a second language, but not much evidence that age plays much of a factor for people of different ages who had the same degree of exposure.