May-December relationships are a common element in many traditional romance stories. They are also quite common in yaoi and some M/M although M/M tends, on average, to use older characters.
That is, yaoi--like traditional romances--tends to pair older lovers with younger ones. M/M tends to pair up men in their mid-30's and 40's. (It's very refreshing.)
The May-December relationship is often criticized or at least joked about. In Friends, Monica's relationship with Richard becomes fodder for humor--however, since Richard is Tom Selleck, everyone decides, well, he's obviously great, so what's the fuss?
In Becker, Reggie's brief stint with a younger man is used to point out the double standard; when Becker dates a much younger woman, he is teased yet praised. Reggie is simply teased.
In real life, men and women in long-term relationships tend to fall within 2-3 years of each other. As women age, they do date younger men--and yes, men are still more likely to date younger women. Still, the 24-year difference between President Macron and his wife is a total outlier.
Gay couples are far more likely to produce an age gap. In Hollywood, James Duff and Jim Nabors married men more than 10 years their junior.
However, even the age gap with gay couples is becoming less common. People tend to marry the people they know, and the people they know tend to be people in their age bracket. After all, Becker breaks up with the younger woman despite the strokes to his ego.
Despite the real-life rarity, December-May couples crop up in romances on a regular basis. Do they work? Are they believable? Are they too, too problematic?
They work if . . .
One or the other member isn't looking for a "Mommy" or a "Daddy".
Let's be real: in real life, sometimes people are looking for a "Mommy" or "Daddy" even when they marry someone close to them in age.
It's unsettling to read about. And a relief when it is left alone. Akihito in Finder--who is close to 12 years Asami's junior--appears thankfully to have zero familial hang-ups. Asami in no way makes up for a father figure that Akihito has lost. Asami irritates Akihito because he is Asami, not because he is "Dad."
Money doesn't become a bullying point.
Let's face it: older people tend to have more money than younger people (though not always). And money is power. It also leads to things like the "Cinderella Complex" whereby one member either anticipates being made a pet of--or takes it for granted as a kind of right.
Far too many badly written romance novels dissolve into the rich older boyfriend buying goodies for the poor younger significant other. But remember Jane Eyre! When Rochester attempts to do the same thing before their disastrous first wedding, she resists. I maintain that psychologically, Jane was ready to run long before any revelation of a crazy wife in the attic. She was feeling utterly smothered by Rochester's pampering. She liked arguing with him, not dressing up in his family jewelry.
Everybody accepts his or her role in the relationship.
I argue elsewhere that dysfunctional relationships can work--at least, relationships that appear dysfunctional to everyone else--if the people in the relationship are okay with the dysfunction. A man who wants to marry a "Mommy" who looks after his needs, soothes his ego, and acts as his personal sounding board (she solves her problems on her own time with the help of her girlfriends) doesn't appeal to me--but there are women out there it would appeal to. Problems occur when either the man doesn't realize that a "Mommy" is what he wants--or when the woman decides too late that she doesn't want the role she's signed up for.
But as long as everyone in the relationship is appeased, what's the harm?
A good romance novel fills in the blanks so the couple complement each other. But only because they want to! Agatha Christie was quite good at creating couples where the weak-willed man actually wanted to be controlled by his domineering wife. If he didn't . . . or didn't know that he did . . . that would be a problem.
So the December-May relationship will work, so long as the December doesn't mind swapping stories with someone who doesn't remember the Cold War. And the May isn't thinking, "I've got to get my aging Lothario a home care worker--when did I sign up for this?"