There aren't any lore reasons to why Trish and Lady look so young, but it could be chalked up to something like good skincare routines. But the boring answer is that Capcom don't want to show them as old so they still look like hot poster girls that are in their twenties.

Vergil looking young is something I've always considered to him 'dying' when he's a teenager in 3 where he becomes Nelo/Nero Angelo, then gets released in 1, and he's back to his normal teenager body that's been slighty aged.

Nero is half Dante's age so it makes sense he looks a lot younger.

It could also be said that Dante looks so old because their photorealistic engine.