I prefer the Japanese 'Chūnibyō', which translates as 'Middle School Syndrome'.
> https://en.wikipedia.org/wiki/Ch%C5%ABniby%C5%8D
Chunnis don't always have the *same* delusions of grandeur, but in the end that's the core of it. And they don't always grow out of it: you can tell by the fact they remain at an eighth-grade emotional level as adults as well.