It's been many years since DMC 3's Vergil was modeled, and he does look admittedly older in the concept and CG art for that game. I think it's fair to say he looks a bit younger than DMC 4 Dante from the footage we've seen, so it's most likely down to them having much better tools to make Vergil look the way they desire now.
If he looks older to you, it may simply be down to the artist's interpretation. Either way, I certainly think he's always looked older than we've been told, even in DMC 3. I mean, there's no way that guy was still in his teens, but I give precedence to Itsuno's intentions.