Predult is a smaller word for, "Prior to being an adult."
It is normally used in an insulting term to show someone who is 18 or over and is not acting their age by being interested in childish toys or ideas, and that instead of acting like an adult, they are acting more like a teenager, tween, or even a child.
(At a party) "Hey look ! They've got boxes of LEGOs in this one room !"
(responding) "That's for their kids. Stop being such a predult and get yourself a beer already."
Little ahole 12 year old who explains why he can hang out with adults because he is almost an adult or as he explains a predult
Hey I am a predult I can hang out with you guys around the fire.
A person who is old enough to have sex but not yet an adult.
The age of consent in the United States ranges from 16 to 18 years old depending on the state, meaning that a person 15 years of age or younger is not a predult.