Lionheart
Solid Ocelot
ok so lets put the age thing aside.we all know that nero has a connection with vergil the blue DT shadow and vergils power quotes so all we can say by this info is that he's vergils son it pretty much seems like he was created for this but capcom messed up.
What I know for certain, is that Nero's blue DT spirit floating around him is Nero's own DT, not Vergil's. If you compare Nero's DT spirit with my avatar, which is his full DT, you can see that.
BUT, I think Vergil's spirit is inside the Yamato (part of it anyway, the part that wants power). Since Nero said that ''a voice echoed: Power. Give me more power'', I think he heard Vergil from a 'distance', so to speak, meaning he heard him call out from inside the Yamato.
Since Vergil's spirit is inside the Yamato, Nero gets a blue DT (kind of like Dante in DMC3, who gets a different DT with different weapons).
Maybe Nero still has to discover his own DT, since with the Yamato DT, Vergil is helping out Nero. The spirit DT is a combination of Nero's DT and Vergil's DT (Nero's devil body, and Vergil's blue aura).
That's what I think anyway. No way to confirm that, though... it's all speculation.