From a writing prompt on our Honors Psychology Discussion Board:
When we think of adulthood, we first think of chronological age. An 18-year-old is an adult, for civil legal purposes. They may sign contracts, marry, vote, drive without restriction, bring a suit in court, and enjoy most of their majority rights. This is not an encompassing freedom, as the state still holds some rights in abeyance, such as the right to purchase and consume alcoholic beverages, hold certain licenses, and run for certain offices. However, in regards to criminal legal purposes, people under the age of 18 (usually at least 16 but as young as 13 in some states) are frequently 'tried as adults', and sentenced as though they were chronologically older. So legally speaking, an adult can chronologically range from 13 to 35.
Other times, when we think of adulthood, we think of scientific age. How old is your brain? For the purposes of car insurance or rental, you're an adult at age 25. For the purposes of booking a hotel room, you're an adult at age 21. Scientifically, your prefrontal cortex, which controls the executive functions including impulse control, planning, and emotional moderation, is not fully developed until around age 25. Scientifically then, you are an adult at age 25.
Maturity is a key factor when considering adulthood. I began working at age 11. My best friend did not learn how to run a washing machine until she was 13. Many of my fellow students still live at home while making school their primary consideration and still come to class late, unprepared, and in pajamas at age 22. Was I an adult when I helped to support a household? Are 22-year-olds adults when they do the opposite? Recent data suggests that adulthood doesn't happen at a single point, but rather, it emerges over time, generally between age 18 and 25. People in this age bracket report that they don’t always feels like adults, nor do they always feel like adolescents. This research aligns with my own belief, which is that our roles in life delineate our status. If you manage adult responsibilities, then you are an adult no matter your chronological age. If you do not yet manage adult responsibilities, then you are not an adult, regardless of your chronological age.