I don’t know you and I don’t know him personally, but I do know of a couple in my family where the husband was twice the wife’s age (18 and 36 when they started dating), and this is where my perspective is coming from. I also am speaking from personal experience here, having met my DH when I had been out of college only a month (at 22) while he was 27 and had been a working adult for several years.
My main concern for you is not that he is a bad man or will purposely hurt you, but that your personal growth will be stifled in marrying someone who is already more or less fully formed as an adult and who has lived a full life for many years. There is likely to be a sort of daughter-father dynamic in the relationship where he is the older, wiser partner, which will impact all sorts of future life decisions, including where to live, how to raise your kids, etc., while you are the one he indulges or spoils. When you turn 30 or so, you might start to feel dissatisfaction with your role as the “little wife” and that is when the rubber is really going to hit the road as to whether your marriage will be a happy one.
Believe me, I am only 5 years younger than my DH, and this happened to me. I had no idea how much I had really lost myself in order to join his universe. We were able to work through it quite successfully, however, and it improved our marriage dramatically. However, with the first couple I referenced, where the husband was 20 years older than the wife, the results were devastating. I don’t want to get into it over the Internet as it is so personal, but basically they had the same problem, except that the inequality was too deep to be overcome, and the wife was the one who suffered.