Do I commit a sin or an iniquity if I remain silent about a truth I know, even though I have reached adolescence, and I fear that if I reveal it, I will be belittled and told: "He is young and knows nothing"?

Enjoining good and forbidding evil is a communal obligation (fard kifaya). It becomes an individual obligation (fard ayn) for someone who has reached puberty if no one else fulfills it, while observing knowledge, gentleness, and patience. If you have not reached puberty, it is commendable for you to denounce evil to the best of your ability, and you will be rewarded for it, and no one should prevent you from doing so.