@Torka don't quote me on it, but I think he was 18 when she was 12, and she's 13 now at this point in the story. He's definitely older than her. Having read the LN I can tell you that they do wait until she comes of age before doing anything that exceeds hand holding (not even kissing, other than back of the hand or a peck on the forehead). They're so pure as a couple it's almost painful to read :-) but that's a good thing because of her age.
Also, this is a country where engagements are often decided by your parents when you're still 5 years old or so. He's apparently begged out of all of them until now, and her original parents would have rather sold her as a slave. The Sergeants haven't gotten around to it yet because she's too new to them, and at this point they would probably have tried to hook her up with Glen anyway.