Uncomfortable

So it's my finances birthday so I'm not going to start anything. But I find this very uncomfortable and awkward and I don't know what else. A little back story. My fiance met this girl at work when she was still 17 and in high school, it's been a couple of years now. He's 28 today and she's no more than 20 still. I just find this, I don't even know how to feel honestly.. I just know it bothers me. Am I wrong to feel weird about it.

P. S. He hasn't commented back to any of the other 30+ people who've wished him a happy birthday.