I think your first suggestion is probably the best one. Lennox's comment also foreshadows Macbeth's encounter with Young Siward, whom we know to be young and inexperienced. In their battle this youth nevertheless fights bravely against the man regarded as the epitome as evil. Macbeth easily kills him and believes this death simply confirms the witches' prophecy that "none of woman born shall harm Macbeth." He doesn't take Young Siward's lack of battle experience into account.
The framing of the story with a contrast between bearded witches and unbearded young men is possibly a technique used by Shakespeare as chiasmus; however, I believe that even very young men join the uprising against the tyrant is the point that Lennox makes in his reference to "many unrough youths." Virtually everyone in Scotland has turned against Macbeth; his own army quickly surrenders when they have the opportunity, indicating they were fighting against the rebels and English forces only because they were ordered to do so, not because they supported the king.