Why do we consider an 18 year old an adult? While you might not personally consider your average 18 year old a full-fledged adult, 18 is a pretty magic number in the US. It’s the the age when you can vote, go to war, work full time, and move out of their parents house. Why is this the case? Well while you might think there’s some sort of biological or traditional explanation, that’s not the case. The age of “adult” in America has always been in flux.