1)
Why does it say "Bachurim v'Gam Besulos"?
1.
Radak: Normally, virgins engage only in adorning their bodies. Also they will praise Hashem!
2.
Yosef Tehilos: It does not say Bachurim Im Besulos, like it says "Zekenim Im Ne'arim", for bachelors are not together with virgins, due to [the Isur of] seclusion, and hearing a woman [sing] is [like seeing] Ervah. However, elders are together with youths.
2)
Why does it say "Zekenim Im Ne'arim"?
1.
Radak: The youths by themselves do not know how to praise. The elders will be with them to teach them.
