Damon Salvatore plays a 23 year old guy. (Ian Somerhalder is actually 31)
Stefan Salvatore plays a 17 year old guy. (Paul Wesley is actually 28)
Yes, there are some cases where actors look younger than they truly are and it's conceivable that they have good genes as demonstrated with Ralph, but my point which I forgot to mention in the previous post is that we should not always rely on stars, most of the time many undergo procedures or treatments at spas (especially the women). Though I admit there are exceptions such as Ralph, Rachel and Alison Lohman, etc.
As for Paul Wesley, he looks more like a college student than a high school guy from the photos I have seen. It's the same with one of the guys from Glee (Mark Salling), he's almost 30 yet he's playing a teen. On the show he does look more like a teen but on red carpet photos he looks more his age, so I'm assuming the clothes, hairstyle, make-up and lighting are working in his favor for the show.