I've mentioned it here and there but one of my HCs is that the reason Dante looks so much older than Vergil in DMC5 is that their appearance mirrors their self perception to an extent, and he thinks of himself as older than Vergil does. Dante thinks he's a worn out old man, so he looks old(er). Vergil on the other hand sees himself as powerful and thus in good shape- and may have a skewed sense of time given how long he was in Hell, which I'd like to say is also evidenced by how young V looks- so while he's not 19, he looks somewhat younger. Or not as worn, really.
The extension of this headcanon is Dante and Vergil essentially aging out of their humanity to an extent, where maintaining their human form becomes an active thing. If Dante thought of himself as younger and put effort into it, he could look younger, and it's probably what would end up happening once he got old enough that a human would die (assuming he wanted to do so at that point). Just like how his demon side is a projection, the human side could be too. Sparda's human form wasn't his natural state. He put that up. So Dante and Vergil could probably do the same. Probably not to the extent of looking 19 again, but they could probably keep themselves looking like they were in their late 20s/early 30s for a long while if they were so inclined.
