My fiance is 76 and I'm 27 but our relationship's real, I'm worried, what should I do?
Age gaps are really common. In fact, they are the rule rather than the exception. For whatever reason in these types of visas, we almost always see an age gap that would be larger than maybe what you normally see. And that goes both ways. When the U.S. citizen's a woman, there's usually a larger age gap with the male intending immigrant, and vice versa.
Most age gaps are not quite as big as 49, but we definitely have seen them before, and they frequently get approved. The key, as with any other holes in your case, is to shore up all the other bona fides of your relationship to build a more solid case for your relationship. This could mean staying together for awhile longer before marrying, it could mean taking a few more trips to see each other in person to establish a relationship, or other things like that. It's not insurmountable, but you will definitely want to bulk up all the other aspects of your case so this age gap issue isn't a deal breaker.