This is a userbox or usercard for people who are denarians. In this case, a denarian is a person whose age is between 10 and 19 years old (inclusive). This includes teenagers and some tweens. Similar terms include vicenarian (20-29), tricenarian (30-39), quadragenarian (40-49), quinquagenarian (50-59), sexagenarian (60-69), septuagenarian (70-79), octogenarian (80-89), and nonagenarian (90-99)
