Romance and ages

What is the appeal behind a young female protagonist falling for an older male protagonists - the ones where the men are over 100 years old, but doesn't look a day over 20 something.I can't give a definite answer to that because I am not getting a degree in psychology or anything related to development. But I just started to find this relationship dynamic interesting.The book I am currently reading, Tiger's Curse is about a girl named Kelsey (16 18y/o, she acts like a kid, though) who decides to go India under the impression she'll help the tiger, Ren, adjust to life in India. But that quickly changes to her trying to break a curse on Ren she met after feeding and reading poetry to him for two weeks. Of course the tiger is not what it seems and he's a prince.He's over 300 years old, and looks young ( no description how old he looks age-wise. Or there is and I forgot, whoops.) Like any book similar to this the female starts to fall in love and/or vice versa.The plots are different but the endgame is usually the same: they fall in love. Maybe there's an article out the that focuses on this relationship dynamic and gives the reason on why it's so appealing to people my age or younger.Maybe it's possible that the book reflects our inner desire to have a older partner or someone old fashion - which some of us wouldn't mind because some people are a-holes.What I haven't seen, which would be interesting is a younger guy falling in love with an older woman, someone over 100 years old looking 20 something. Or even people of the same gender with the dynamic. I bet that would be good. (Hint, hint.)There's nothing wrong the large age gaps, otherwise I wouldn't have it on my bookshelves, but it is a little creepy. Especially if they aren't legal. But that's a huge plot hole for later.