It's the darker features. The darker, fuller eyebrows in particular. He also has a very square jaw. I think the combination gives him an older look. I'm sure he looked older than Shatner even at the same age.My only gripe (if you can even call it that) is that he seems a little old somehow. Even though the actor is only about four years older than Shatner was, he comes across a fair bit older to me.
It doesn't help that he doesn't compare well physically to either Mount or Peck, who are both in fantastic shape.
Unfortunately, I also see the Jim Carrey resemblance. I don't hate Carrey, but it definitely takes me out of it to see that.
I'm hopeful that he'll grow on me more with greater exposure next season.