We had an entire tv show called "Younger" because a woman in her 40s pretended to be 25. The Night is young, it's 8pm at night, there's only so many hours left in the day. Just like Teen Titans can vary in age between 13-19. 18 is considered an adult but you are still a "teen"
No one looks at someone in their early 20s as "old", they are still referred to plenty as being young.