An answer on this question suggests that if a boy fathers a child before he is bar mitzvah (which is physically possible), then he fulfills p'ru ur'vu even though he wasn't yet commanded. Is that a general principle? I am particularly asking about the "once in your life" mitzvot, not the things you do regularly so there will be plenty more opportunities later.
Suppose a minor writes a sefer torah (by donating money he earned to help pay for one), and this is the only time in his life that he does so. Or, suppose that minor fathers two children before bar mitzvah and no more. Are these people yotzei? Does an action performed before you were obligated filfill what you will later be obligated in?