A "responsible adult" is someone who is mature, trustworthy, and accountable for their actions. They make good choices, take care of themselves and others, and can be relied upon to make responsible decisions. Full definition
Researchers have found kids who have authoritative parents are most likely to become responsible adults who feel comfortable expressing their opinions. (verywellfamily.com)
Give them the independence they need to grow into responsible adults who know how to take care of themselves. (childdevelopmentinfo.com)
And make sure these are supervised by responsible adults. (thespruce.com)