Affectation

[Raeanne Dimick]

An attempt to assume or exhibit what is not natural or
real; false display; artificial show.

[Britney]

Something you do to make yourself seem [cool]er than you are. A lot of the time you dont really realize youre doing it, but all of the time you look a fool to everyone watching, assuming theyve grown up more than you have.
Almost everyone has some affectations, but people with a lot of them stick out like sore thumbs and they dont even know it. See [try-hard].
People in high school and college (especially) tend to have a lot of affectations because they are stupid. Or because they dont have the life experience to know that they are being stupid. Oftentimes both.

[Dianne]

behaviour or speech that is not sincere: