Ignoring the fact that age of consent varies from location to location, I'd kind of doubt that Vergil would care.If Vergil was of legal age when Nero was conceived (less squicky that way),
Devil May Cry is pretty anime man.Dude Nero looks nothing like a sixteen year old. Stop with the anime goggles here where a 13 year old looks 21. If anything, Nero looks about 19 or 20.
Anyway, in the recent interviews they talk about how Nero's attitude in DMC4 was akin to that of a teenage highschooler trying to impress his girlfriend. So I seriously doubt Nero was intended to be in his 20's.