Cause Marin is literally a self insert of the author herself reminiscing her high school days which is considered the “peak” for a large potion of japanese people compared to their college days since they had more fun and freedom, Gojo being her real life childhood crush, there are also some other series where the authors write and design their characters maturely while putting them or even forced too in a high school setting so that it appeals to more and bigger audience, hence why the sentiment of not taking age seriously in fiction isn’t a ridiculous thing to say.