Is the delay in the appearance of the beard and mustache at the age of eighteen, and a person's appearance looking younger than their age, due to committing forbidden acts and then repenting from them?

You are still young, and your beard may grow later. If it does, honor it by letting it grow. If it does not grow, it is a minor matter, for a beard is not proof of Allah's love or the strength of one's faith. Rather, it is among the innate characteristics in which people differ, and Qais ibn Sa'd did not have a beard. Know that sins are wounds that can be fatal, corrupting the heart and placing a barrier between it and repentance. Therefore, strive to abandon sins through righteous companionship, abundant acts of obedience, and seeking refuge in Allah.