I thought beardless=unmarried in most cases, not youth, per se. St. Demetrius of Thessaloniki was 36 when martyred and St. John the Russian was in his 40s but both are portrayed beardless. Of course, St. John the Theologian is bearded in the icons in which he is writing.
... but none of these were clergy. St John is perhaps the only saint who has two distinct iconographic standards - the youthful apostle, and the balding, bearded old writer of the Gospel and Revelation.
On beardlessness signifying unmarried:
In many cultures, including the Middle East and the Mediterranean, a boy was deemed to be a man when he was able to grow facial hair. He did not need to marry before growing a moustache or beard. This held until only the last couple of generations, when the norm of clean-shaven men became common. There are various words in these cultures which equate the lack of a mustache or beard with immaturity or being unmanly.
And, of course, Orthodox clergy are bearded, including the monastics.