That's because it's not a typical sport with a normal grooming system. In other sports, children with talent are groomed for success. By the time they're in their early 20s they're practically a finished product after years of training and competing. No one is training and competing in MMA from childhood. At best they're doing another combat sport and trying to become an Olympian or World Champion in that particular discipline. Most guys fall into MMA in their late teens which means that they become a finished product much later in life hence this perception that MMA athletes peak