Well, consider that Lady was around 19 in DMC3, same as Dante. The developers said that DMC4 takes place around ten years after DMC3, which would make Dante and Lady 30. They look like it too, to me. If they were forty, at least some wrinkles would show. Dante's not immortal either, like some people think.
I see no possession by Nero Angelo in Nero. He does get a devil trigger that looks blue and has a sheath for the Yamato, but other than that, it looks nothing like Vergil's DT or Nero Angelo. They both didn't have a halo attached to their shoulders, and their bodies were pretty different. Even the horns don't match Nero Angelo's horns or Vergil's DT horns.
I think that either Vergil's soul helps Nero out when he goes into devil trigger, OR the Yamato simply grants Nero a blue devil trigger because it did the same for Vergil -- they are both of the Sparda bloodline, therefore they get similar DTs. Seems to make sense.
Nero, as far as I know, was never human. Yes, the booklet says he is, but I think that was just meant as build-up to the realization that he's partially demonic. Nero has always had white hair and extraordinary power, he was left at the orphanage by an unknown person, Vergil was in Fortuna at some point, Sparda was the feudal lord of Fortuna... it's all a bit too coincidental. Nero even fights Dante on the statue of Sparda, and throws Sparda's sword at him. Berial says how similar he is to Sparda, and that he's not human. Sanctus even says Nero is a 'descendant of Sparda's blood', blood probably meaning 'family'. He also says Nero 'inherited' the power of Sparda. Can't inherit something from Sparda if you're not related to him.
As for the Devil Bringer, the DMC4 booklet says that he was attacked in Mitis Forest and got an injury to his arm. The DB came into existence due to that injury, which makes me suspect it's simply an incomplete Devil Trigger. After his arm changed, he started hearing Vergil's soul call out from the Yamato -- and the lab where the Yamato is, is actually pretty close to the forest. So that explains why he heard Vergil (or possibly the Yamato itself) cry out for power. Remember, the Yamato is the Rebellion's counterpart, and it's possible those two swords contain parts of Sparda's soul. The Rebellion even awakened when Dante got his DT.
Anyway, maybe Nero couldn't control his DT, so he only got the arm. I don't think a parasite is in his arm, and I don't think it's Nero Angelo's arm either. It's blue and red, and has powers Nero Angelo didn't (and shouldn't) have. It's weird that Nero has these ethereal powers, since Vergil never had an overpowered stretchy arm, nor did Sparda as far as I know. But maybe it's just a random something that could happen in people who get partially possessed by other demons.
No one at Capcom has every confirmed their ages. It isn't in any of the books, manga or video games. Their "age are rumors. Demons apparently don't wrinkle haha so he could be 29, could be older. I always pictured him mid-thirties in 4 but everyone sees it differently. The only person whose age has been hinted at is Nero, to be in late-teens to early twenties, but at the end of the day, they are still just hints, nothing concrete. But it has never been said Dante, Vergil, and Nero are X age. With the exception of they were 8 when Eva died. Unless somebody at Capcom decided, years after all of the games, books, and mangas have been out to suddenly state their age.
The developer in some interview said that DMC4 takes place ten years after DMC3. Unfortunately, the internet doesn't seem to care much about that interview anymore, can't find it anywhere. But I assure you, I remember reading that interview back then. Since Dante was 19 in DMC3 (I'm pretty sure that was confirmed), Dante must be somewhere around 29 to 33 in DMC4. If he's any older than that, it wouldn't be ''around ten years''.
In DMC1, Dante is 28 - that's actually explicitly stated by Trish: ''you're the man who lost a mother and brother to evil twenty years ago''. Since it's known Dante lost Vergil and Eva when he was eight, he must be 28 in DMC1.
In DMC2, his age is unknown, though of course it takes place after DMC4, making him at least in his mid thirties or maybe even forties.
As for whether he's Vergil's son or more related to Sparda, it could be anything. Vergil doesn't even have to be dead, he could've been freed from Mundus's control after DMC1. It's just the blue devil trigger Nero's got, and his Vergil quote of ''Power. Give me more power!'' that throws people off. That, and the fact that he says things like 'jackpot!' and 'blast' when he fights in devil trigger. But those quotes could just be ways to show how similar he is to Vergil, doesn't necessarily mean he's possessed by Vergil or that he's his son.
There doesn't seem to be any real evidence either way -- but yes I know there's an art book that says ''he really is Vergil's son''...still not proof and could just be Bingo Morihashi's original idea. That said, I do think that all the references to Sparda in DMC4 are food for thought. Berial saying ''you are just like he was (Sparda)''. Fighting Dante on a statue of Sparda, throwing Sparda's sword. Getting a purple aura for a split second when he takes the Yamato, having slight purple undertones with the Devil Trigger. Hell, his DT concept art even shows him with a full DT that's got some kind of glowing eye patch, but Nero's eye isn't damaged. Sparda had a monocle, so I think it's another reference to Sparda.
. although it never really clear where Sparda is in the original series, they just say that he vanished but i like DmC's idea that he is imprisoned in hell....if they can put this into a game that would make for a great sons and father reunion 
. it would've been a wonderful moment in the series though