The age difference is simply not large enough for Nero to be Vergil's son. Simple as that.
Seeing as the "ages" were never taken seriously in DMC, personally I wouldn't go with that as proof. After all they say vague things like "Dante is
about 18-19 in DMC3" and "DMC4 is
about 10 years after DMC1". If they released an official thingy which stated the exact ages seriously,
then we'd have something to work for. But using these claims that the creators said in interviews, makes me think they just thought them on the spot, for example: thinking that Dante should be a teenager in DMC3 yet old enough, so let's say he's about 18, and it'll work out.
Don't get me wrong, I'm not a fan of Nero is Vergil's son-theory - though the DMC4 novels supposedly supports it, therefore maybe that's what the developers had in their mind while creating the game - but for
me the
ages are so messed up that they can't be considered definite proof. Stuff said in interviews are fine, but stuff said inside the game/or just the manual that comes with the game are
the proof.
Then again, even in that regard, we do see lots of contradictions - maybe that's one of the reasons for DmC trying to fix all that.