1)
Who are the "Ne'arim"?
1.
Rashi: The Bavliyim, who are Menu'arim (empty) of Mitzvos.
2.
Radak: Resha'im of the world who get stronger each day, like youths. When Hashem wants, they will tire; they will not have even the strength of youths and bachelors.
3.
Malbim: Those who are strong now.
2)
Who are the "Bachurim"?
1.
Rashi: Those who are now strong.
2.
Radak: Resha'im of the world who are complete in their strength, like bachelors. When Hashem wants, their strength will not help; they will stumble.