There's also the physical appeal; even someone who'se normally very confident might feel the need to go to extra measures to appear more fit and even youthful, out of fear of loosing their younger partner at any point. It could create a complex for the older person, while the younger person could pick up on vibes of insecurity and loose respect for their older partner.
Then there's life stages; in her 50's, Lisa Bonet for example was much more complacent to stay home and work on herself, where Jason, in his forties, still had a lot of partying to do. Jason would often send pictures of himself to her from wherever he was, having a good time. (Really sounds more parental). Every new decade makes a person have to re-evaluate what's important, based off slowing down to balancing physical health, mental health and spiritual health. Even if we're young at heart, the body breaks down. We're kinda forced to re-evaluate our life style and become more tempered and complacent with smaller things.