hmmm another good point you observed, i didnt think of it till now, . . that um maybe, they 'last' because of the lack of peer pressure impose on them as couples, like they dont have to attend each other relatives' functions, like weddings, to face dissaprovals?. But then again, how they do it like go to weddings on their own? (suppose they might, and may bang others as a fling with the 'same age' then come back together talking about their adventures?!
i gotta stop this wild imaginations lol, but im just saying it sure a mystery about how the odd age-gapped couple lasts, gee maybe plainly they just in love...and with minus the added pressure to please everyone with they differently budgeted wealth or ideas of what to wear, i dunno...i guess there's a whole lof of factors, combinations or just plain luck
another idea is, the younger have intense interest in say art history, no one around same age reciprocates but the older freind-turned lover may share it, and indeed they may take it further to explore their fantasties (not purely sexual) but more like the ideas of romanticism (yes sex in there) but the whole thing is like intellectual/life passionate kind of exhilieration, a way of how the bond gave them an oppurtunity to LIVE like that, i mean actualise their hopes and dreams. Both old and stale are both 'somewhat' sharing the 'im over the hill feelings' saps out happiness, whereas the both young and fresh but with no idea how long the fresh will stay (like breads straight out the bakery - not in plastic bags) the youngs are havent got the 'older teacher/young student type chemistry' so they both are unsure and still have questions and yet they havent discovered whats their real passions are, and in 'pressure' to hold face in front of the 'lots of freinds' only to find out they only themselves in part of the whole 'game' as sports-the walking-talking maniquin pieces LOL and not 'characters with history books under their arms ready to show and tell. yeah i dunno , but i hope this attemp to describe the why's here isnt toooo absurd!
Dramas movies sometimes have this themes and they are interesting to watch, especially the british films.