I have a completely different view of the whole thing!
I always thought that because Dante was half demon, he aged at a different rate as humans do. I mean, DMC3 could easily have taken place in the seventies, and DMC1 taking place 10 years later in the eighties still would have made some sense, as there aren't too many anachronisms. After all, the only things to suggest that the era is relatively modern (as in the last 25/30 years or so) are the guns, Trish's sports motorbike and the red bi-plane. The rest is all arcane and magical.
I mean, Sparda defeated Mundus the first time around 2000 years ago, yet still has managed to have two youthful sons living in the modern day, as well as there having been enough time since his death for his story to become a legend, and have an entire religion grow from that legend and have statues and churches erected in his sanctified honor.
So unless in some future DMC prequel where we learn that there's some sort of time travel involved between when Sparda had his children 2000 years or so ago, to explain how those children are still youthful in the modern day, I'm going to assume that Dante/Vergil/Trish are ageless/age at a much slower rate than we would initially presume.