Capcom seems to willy nilly on how fast cambions age in the DMC universe, but it's shifts between something similar to normal humans and slower than normal humans. Given Nero's age, (between 17 and 19 at youngest), and the fact the twins were 19 in DMC3 as stated by DMC3's director in a production interview, with DMC4 being about a decade later, Nero's too old to be Vergil's son. Now if it were a figuritive or metaphorical "son", like he's the heir to Yamato and succeeding Vergil to a degree, that's different. But Morihashi Bingo clearly wanted badly for Nero to literally be Vergil's offspring, and it really just does not work on so many regards.