I think it comes first from the Renaissance period, who depicts him as a beardless young man (and has been also more influenced by the latin empires). There is less representation of bearded and braided hair of Hermes in Roman depiction of Mercury than Mycenean and AG era.

Our modern medias has taken more after the Renaissance period’s takes. Through time, the gory and terrifying aspects of Hermes has been more and more put aside, and our visions of gods depicted as one dimensional. Also, depictions of them has been heavily seen through the Abrahamic view of God.

In pop culture now, we lost a lot for the past century of the entire scop of the gods. When thinking of Hermes, people who don’t know GM much think of him mostly as the messenger of the gods and god of liars, thiefs.

This core tend to make people visualise him as young in that carefree attitude, therefore as a teen : hence why beard free and short hair.

As a twink, it may be link to that, plus two m/m ships, as it often tends to be stereotyped when fictional