Rick Riordan Never Expected the Longevity of Percy Jackson 19 Years After the First Book: 'Mind-Boggling' (Exclusive)

When Rick Riordan starting writing Percy Jackson novels, his son was just a middle schooler. Now, he's in his 30s.

The acclaimed Percy Jackson series author released the second book in his newest trilogy, The Senior Year Adventures, on Sept. 24. Titled Wrath of the Triple Goddess, the book, which is the seventh overall in the saga, picks up where the sixth left off, as Percy battles to get three recommendation letters so he can join his girlfriend, Annabeth Chase, at New Rome University.

The first novel, Percy Jackson and the Lightning Thief, was published in 2005. That means, as Riordan tells PEOPLE, the first generation of middle-schoolers to discover the series are now adults, many with children of their own — including his son.

Percy Jackson, a demigod with ADHD and dyslexia, was born out of Riordan's own son's struggles with both and the bedtime stories the author told him, to make his son feel less alone. The story has now permeated pop culture, leaving a lasting impression on generations of younger and older readers alike, with movie adaptations and a TV show currently streaming on Disney+.

"The fact that this little story that I told [my son] is still with us and kids are still picking it up for the first time today, and there are readers now who read it and are old enough now that they're starting to have kids of their own — I mean, the whole thing is just mind-boggling," Riordan tells PEOPLE.

He continues, "I never saw this as something that would have this kind of longevity. I was just writing the best story I could for my son at the time that I wrote it. And it's been amazing beyond any expectations I had, the kind of longevity that this has had. But it's hugely gratifying. It's an honor."

Though the books can appeal to readers of all ages, the target audience has always been for middle grade readers. And even though the world has changed quite a bit from 2005 to 2024, Riordan hopes that some themes are universal.

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"I do have to hope that kids are kids and 12-year-olds are 12-year-olds, whether it's 1960 or 2024. Generally speaking, people are people and they're going to be worried about the same stuff," he says. "It may express itself in a different way, and it may be about different things that are going on in the world, but we have these universal themes that just keep cropping up."

For one thing, Riordan tells PEOPLE he intentionally keeps Percy young, and the era in which the book takes place remains ambiguous (though he did include a mention of Beyoncé's 2024 anthem "Texas Hold 'Em").

However, in Wrath of the Triple Goddess, Percy is definitively in the later years of being a teenager as a senior in high school, balancing not only the trials and tribulations that come with being the son of Poseidon, but also everyday worries like studying for tests, getting into college and his relationship with Annabeth.

Knowing Percy was getting older and his relationship is starting to mature, Riordan says he based their love off of his own relationship with his wife, whom he also started dating in high school. Writing their love story "felt very natural," he says, but he also knew he had to have them experience more mature problems than they did when the series started, like what would happen when they graduated high school and started college – a very relatable problem for many teens.

"Percy would be stressing, 'Oh my gosh, I don't want her to go to one college and have me back here working at the corner store ... I really have to step up my game so I can stay with her and we can stay on track,' " Riordan says. "So it just seemed relatable to me, and I just wanted their relationship to be evolving in a way that made sense."

"They're still kids, they still got a lot to learn, but they're trying the best to figure it out together," the author adds.

Nineteen years, two movies and one TV show later, Riordan is still always looking for "new angles" and fresh stories in Percy's world and the other mythological series he's penned. But his intention has always been the same: to make a difference in people's lives, even if that's as simple as putting a smile on their faces.

"I'm always sort of conscious of that responsibility too, in the back of my mind that I have to do my best to make this a story that really stays with people and is helpful in some way, even if it's only just for a moment of escape," he says.

Percy Jackson and the Olympians: Wrath of the Triple Goddess by Rick Riordan is available now, wherever books are sold.