It's when you enter official adulthood for the first time, gaining great privileges along with serious legal implications. At the age of 18 you are legally considered an adult in nearly every state in the union. An year-old can also buy and sell real estate and stock, inherit.
Legal Adult(s) were not considered as such until they were 27, then 25, 23, 21, 20, 19, and currently It has to do with the maturity issue. Hence why you can.
Just about everything in all states flips to "adult" at 18 - contracts, etc. Obviously that's not % universal - alcohol isn't enabled until you're