I thought Nero was 16/17. So if anything Vergil had a kid at the age of 13/14.....that is if Vergil/Dante is 29/30 by DMC4.
Also...I don't buy the time gap between DMC1-anime-DMC4 statement by Kobayashi being only 1 year.
Here is why:
Lady and Dante....both seemed to have changed a lot in 1 year.
Mostly its Lady.
In the anime, she barely looks that much different from her DMC3 self and that was.......9-10 years ago.
Yet DMC4 Lady looks much more maturer than anime Lady.
Also, Lady still has that scar on her nose in the anime.
But in DMC4 its gone.
How come a scar she had in DMC3 still be around 9 years for the anime but disappear in 1 year for DMC4...unless if she was using skin products or make up.
Then there is her breast size. Her breast size in the anime seemed to be the same from DMC3.
Meaning that Lady didn't mature much over the course of DMC3 to the anime but she matured a lot from the anime to DMC4.
Her breasts grew a few cup sizes in a course of 1 year....maybe less but remained almost the same from when she was 19 in DMC3 (???) to 28/29 in the anime.
Plus the 10 year comment came from Kobayashi...just one guy who works at Capcom. I mean at this point Kamiya, the guy who made the first DMC game and the creator of DMC, can say anything about DMC and it will get shot down by Capcom.
Nothing in the games, anime, or other source confirms the time gap between DMC1-DMC4 is 1 year. Just one comment/statement from one person who technically isn't the proprietor of DMC (that would be Itsuno). Capcom can literally come out and now state (in an artbook maybe) that Kobayashi was wrong, the time gap between DMC3 & DMC4 is not 10 years. If anything it seems making Nero the child of Vergil was an idea Kobayashi was against and did everything in his power to not make it possible in regards to the story (making statements that aren't really backed by the game itself). Bingo was most likely for it and left Capcom to write his own version of DMC4 since Kobayashi was being a prick.....I wouldn't be surprised.
But I could be wrong and maybe Itsuno also said the time gap between DMC3 and DMC4 was 10 years....but I don't recall that.