The problem with these type of relationships is the age gap caused power imbalance. While two people in their same age category (like 22 and 26 for example), they have the near same experience with life which creates the base of their personality. In a relationship with big age gap there is a good reason to assume that the older partner somewhat used their own extra experience to groom the younger one, often emotional or monetary manipulation, physical violence or other abusive methods. I'm sure there are honest relationship out there between older people with younger soul and younger people with more experience but that mostly isn't the case. There are plenty of examples in anyone's environment to tell that the former more likely. Big age gap is therefor aren't fully concensual.