Do you expect people to have sufficient cognitive capacities by the time they are 18 to choose their language? ¿Hablas español? One could very well argue "yes

Do you expect people to have sufficient cognitive capacities by the time they are 18 to choose their language? ¿Hablas español? One could very well argue "yes", but first and foremost speaking the language of their offline communities is pretty likely, if only through sheer inertia and a lack of concrete reasons to question that choice. At 18, one stands a very good chance of having spent one's entire life roughly within the confines of a single monoculture and it's morals, social pressures, religion, language, etc etc etc. - quite possibly with contact intentionally limited by one's parents (not all of us were blessed/cursed with unrestricted, unmonitored, and unfiltered internet access, and many parents do a lot of meddling when it comes to friends and other social connections as well.) It's often shortly after, when said people go to college, that they experience their first true culture shock. To the point where some communities will outright discourage college (pressuring boys to inherit a trade, or girls to marry and become homemakers), or at least discourage going out-of-state, pressuring children to stay closer where they can be - if not controlled - at least kept in a similar social environment, where they obstensibly won't "lose their way", where they can be more readily influenced. Not saying one can't break away at 18, but for many people, the question isn't "Why [become a mormon missionary]?" but "Why break with tradition, family, friends, and one's entire support network, just to skip a little missionary work?"