What age range does shonen cover exactly?

[GenkaiPower]

I wonder as was reading a light novel where an 18 year old about to graduate is described as shonen
but in another manga an 18 year old was called seinen
So what age range does shonen cover? and if used for someone 18 can it mean young adult?

 

[johnnyG]

Juvenile, but one twist is 特定少年 (legal term), which is 18-19 and legally adult but a 2022 law still classes them as juveniles.

青年 is young man (I'll have to ask my wife and kids if I'm 万年青年. )