In my opinion, with Ian and Nina's current relationship, age makes a big difference.
First of all, let me just clear things up. All I've heard for the moment are rumours and even some gossip, about Nina and Ian's current relationship status. None came out from their own mouths, as far as I know, but the tell-tale signs are strong enough to leave me biased.
They are clearly in a relationship.
Since that's settled, let me continue to my main topic.
Nina Dobreva, popularly known as Nina Dobrev is a young and beautiful, 22 year old Bulgarian-born actress with a knack for acting and fashion. Ian Somerhalder, popularly known as Ian Somerhalder, is a not so young 32 year old American-born actor with a knack for acting and one of those rare cases who could pull off a side-smile ridiculously well.
Both came from a humble background in acting. Nina wasn't your typical Drew Barrymore who was popular ever since she was a little girl until now or Ian as Macualay Culkin, who's not doing so well after his stint with the world of drugs and alcohol. So children, don't go near those things! They had humble beginnings in the faded backgrounds of the acting world before both stepped into their famous roles as Katherine/Elena for Nina and Damon for Ian in the popular CW show, The Vampire Diaries.
Since their status boost after being casted in the well-written show which is now popular all over the world, it was no question for people to have the natural curiosity to ask the infamous question if either of them would date their co-stars, ahem, with each other. Nina of course said no and even backed it up (paraphrased, of course) that it might just be a little too weird to date your co-stars since if the relationship goes kaboom, the getting along with each other part would be a total nuclear explosion.
I believe Ian's had his fun but you know, there's some late bloomers now and then but my fingers are crossed that his loving looks to his co-star are true. Nina's a beautiful girl who deserves someone to take care of her. You can see it in her eyes and with her smile.
Love, love.