Age gap is one of the OG romance tropes and has been around for a long time. You can find evidence of it in early historical romances, written back when the heroes and heroines often had decades separating them. Sometimes, the heroines were younger than 18 when they got together with the heroes.
Now, most age gap romances follow an unspoken but nonetheless important rule—the year gaps can be huge, but the youngest character has to be over 18. Preferably, over 21. It's what most readers expect when they pick up an age gap romance.
And there are many, many readers of age gap romance. The trope has exploded in the last few years and has been a near constant on the bestseller charts. There are age gap romances in every romance sub-genre, but the most common sub-genres to feature the trope are contemporary romance, dark romance, and mafia romance.
And why is this trope so hot right now?
One word: Tension!
The age gap trope is such a good way of creating that juicy tension that keeps romance readers turning pages.
Tension arises from conflict, and in this case, it's internal conflict, aka what your characters have to overcome emotionally before they can find their happily ever after.
For example, one of your main characters might worry about the public perception of their relationship.
Or one of your main characters might struggle with the idea that they are compatible with their love interest despite the age gap; maybe they worry that they're at very different stages of life and don't have enough in common. That's one of the internal conflicts in Penelope Douglas' contemporary age gap romance, Birthday Girl. I loved seeing the characters in that book struggle with the repercussions in each other's lives if they decide to commit.
Solving internal conflicts like these requires character development, which is the backbone of so many good romance novels. And it's one of the reasons readers love age gap romances—they get the satisfaction of seeing two people grow through their love for one another, a love that defies expectations.
The growth can be subtle, like in The X Ingredient by Roslyn Sinclair, where it's in Diana that we see the most change, as she grapples with her sexuality and divorce, or more overt and equal between the characters, as in Birthday Girl by Penelope Douglas, and To Tame a Wicked Widow by Nicola Davidson, an older woman, younger man historical romance with characters who have to reckon with family and societal expectations to be together.
No matter what sub-genre of age gap romance you write, keep that tension and internal conflict in mind, and don't be afraid to combine age gap with other tropes.
It goes particularly well with guardian/ward scenarios, where the age difference adds another layer of internal conflict for the guardian, who has to battle feelings for someone who is not only younger than them but is also under their care. The trope also works beautifully with forced proximity, because the characters can't escape their feelings for each other and have to eventually confront them. Cole McCade's Over and Over Again is a great example of this trope combo.