To be fair, I wasn’t told by anyone at school or home to do so. I saw a sign in the local post office describing it while I was an earlier teen. Even that was somewhat coincidental, as our family never came into town to get mail, as we lived in the country.
Strangely, there is no entity I’m aware of that is required to assure this information to each young male.
That said, you might be able to contact 0bama and see how he got his Selective Service post card accepted just prior to the election.
I recall thinking around the time I was turning 18, that there wasn't any official communication from the gov't telling me that I was required to register for the Selective Service. I only knew through word of mouth.
Now, Vietnam was still raging at that time, so "word of mouth" about this was very prevalent among guys my age.
Still, a guy could probably have just not signed up, then claimed ignorance if the authorities ever hassled him.