Teshuvah, commonly translated as "repentance," literally means "return." Teshuvah, a central theme during the High Holiday season, is an opportunity for a person to realign themselves to their goals and return to what's important.
Saying sorry, apologizing to friends for mistakes, and developing empathy for how other people feel are accessible ways for young children to practice Teshuvah. It's never too late for a person of any age to do Teshuvah - there are no time constraints or limits.