So, I've been thinking about this for a while. My theory is that Saiyans can't grow facial hair until WELL into their prime. The only Saiyans we've seen with facial hair are Nappa, King Vegeta, one of the guys in Bardocks squad (and Paragus, in non cannon). They all appear to be older, experienced Saiyans. It's fair to say the all Saiyan males are warriors (or a very large percentage). As a result many would die fairly young.

This would mean that having a beard or moustache is a visual indication that the Saiyan has survived many battles and should be respected in that culture. It would also explain why Vegeta was so proud of his moustache in GT. (So, just humour me, and let's say GT is a thing.) Vegeta is older than Goku, and therefore can grow a moustache where Goku cannot.

Just a thought, what do you folks think?