This I remember.
The earliest painted depictions of Jesus by Christians were beardless. If you look in the catacombs and in other Christian art of the period, Jesus is always a young man without a beard. Considering that these were made prior to the formal canonization of the New Testament, the Jehovah's Witnesses saw this as evidence that Jesus was beardless. Why?
Foremost was the time period. After WWII when the Western clean-shaven look in men was synonymous with moral virtue, this really skewed the way the Witness leadership saw itself and the world around them. Those at the time who wore beards were synonymous with free-thinking, rebellion, free-love...and communists! Communists leaders wore (you guessed it) beards! This played a big part in the rejection of facial hair after WWII. Also before the hippies of the 1960s, there was the beatnik of the 1950s, and before them the Bohemians of the 1940s. None of these styles represented the Western Allies and the virtuous Christian, freedom-loving image that went along with it.
The JW leadership of the period after WWII ate this U.S. white-man stereotype up (remember, commies wore beards, and they were against freedom and God--icky poo!). And these JWs were also, like today, quite ignorant of what you learn from a full basic education. While these beardless depictions of Jesus were indeed the earliest produced by Christians, they were not meant to be depictions of how Jesus actually looked.
In ancient times artists painted stories using stereotypes and symbols which were universal in the Hellenistic and Roman worlds. Incarnations of gods (when male) were depicted as young men. A nimbus (halo) was a symbol of unearthly glow or honor. Leaders were often pictured as shepherds.
All these types of iconography were employed by Gentile Christians in their earliest portrayals of Jesus Christ. Since they believed Jesus was an incarnation of God, the way to paint this was to illustrate a young man. Young men, by nature, had no beards. This was the reason you see Jesus illustrated like this in paintings of this period.
Being as ignorant as they've always been, the JW leadership knew nothing about ancient iconography and art. Interpreting everything from their WWII point of view and being Western, white American males, they interpreted these early pictures without doing their homework: "These are the oldest pictures of Jesus. Jesus is beardless. Since he was a perfect man, perfect men must be beardless. Let's illustrate Jesus as beardless in our publications and forbid beards."
They did eventually admit their mistake, but did not do away with what caused it, namely interpreting things from their limited perspective and failing to test their conclusions by having them verified by disinterested parties.