Doing too much math in my head at 1am to figure out Nero’s age because there’s so much different information. But following the timeline of Dante’s and Vergil’s age with what we do have confirmed age wise for them, for it to make sense Nero would have to be 18-19 in DMC4, despite Deadly Fortune (by Morihashi) taking place slightly before DMC4 saying he was 16-17 (although there is also a part where I think Sanctus calls Nero 17-18, but translations are always weird so I’m not sure really). Yet, Itsuno also said he was the same age as Dante and Vergil in DMC3 (19). Yet, that was also when the timeline was still 3142 before it was changed to 3124 when DMC5 was coming out/came out.
If Dante and Vergil were 19 during DMC3, that means Vergil was in Fortuna around ages 17-18 which was when Nero would have been conceived. So, when Vergil is 19, Nero would have likely just been born or born when Vergil was 18 (which, if DMC3 takes place on his and Dante’s 19th birthday, it’s more likely Nero was born around when Vergil was 18-19 rather than 19-20).
Flash forward 9 years later during DMC1, Dante and Vergil are 28 (since it had been 20 years since their mom dad and that happened when they were 8). That would make Nero 9-10 years old.
DMC2 & TAS takes place 5 years later (via Behind the Nightmare, although this was only mentioned to me by someone who had read it/gone over it), making Dante 33 and Nero around 14-15. Patty is 8 years old in TAS and in DMC5 she is turning 18. That would make Dante and Vergil around 43 in DMC5. That would also mean Nero is around 24-25.
DMC5 takes place 6 years after DMC4, which would make Nero around 18-19 during the events of DMC4.
If Nero was 16-17 during the events of DMC4, that would have meant Vergil was around his early 20’s when Nero was conceived/born and Vergil was not near around at that point in time. Bro was too busy in Hell to get busy in Fortuna. (That, or DMC2/TAS only took place 3 years after DMC1 and BtN info is wrong).
I could be missing things or even have information wrong, if you know something different you’re welcome to correct me. That’s just how I understood it by following Dante’s and Vergil’s ages to calculate Nero’s, despite the contradictory details on his age in DMC4 and DF (as well as the contradictory details between DF and DMC4).
Anyway, I should sleep lol (this took over 40 minutes to type out fully btw it’s nearly 2am now)