Most of the relationships mentioned so far have been with the man being older. The strangest relationship I came across was years ago when one of the new starters in a club I belonged to introduced me to a lady that I'd seen him with before. The trouble was that I had been having trouble deciding if she was his mum or his gran'. At the time he was in his early twenties and she had to be in her sixties easily. I found out later her son was in the army and was aware of the relationship. If she'd been a "glamourous grannie" I could almost understand the attraction but she really did come across as an old biddy. To be fair, she was great to talk to but I really struggled to understand the relationship on a physical basis. Just seeing them holding hands was strange.
I've just re-read the above and the "Guardian reader" in me says "so what, if they're happy together leave them alone". It still strikes me as odd though.