I doubt Claire is a regular thriller reader, but I could be wrong. She's also not Catholic, but knows Catholicism from her younger days. She's definitely provided some good Catholic feedback.
As for the age range, it's probably not too different than non-Christians, roughly fourteen and up, provided I keep the violence under control. I have one advantage and disadvantage compared to Dan Brown. The advantage is that there is no fictional telling of the End Times from a Catholic perspective, except a book written over one hundred years ago. Many younger Catholics read the Left Behind series and think it's Catholic, which it definitely isn't. Interestingly, Pope Francis recommended the century-old book. It's called Lord of the World. The similarity to my series title is a coincidence, but it will help me on Amazon and other sites given the matching algorithms they use.
The "disadvantage" I have compared to Dan Brown is my book is much more religious than his, with more everyday Catholic elements, including a lot of praying. I had Connor pray silently for the dying cardinal, but I plan to go back and verbalize that, as that is more likely in a setting of so many religious characters in the room. They would want to join in. Those are probably a turnoff to secular readers, but I prefer it that way. One element to the story that will annoy serious Catholics is that I have a gay priest running an orphanage of boys, and the most senior cardinal in the Vatican allows it to continue. I have other elements that some Catholics won't like, such as God the Father accepting a new challenge from Satan that is winner-take-all. Also, the idea that Jesus will return as a boy. Those are not supported by Revelation, even though I found wiggle room in Acts 1 for the latter.
I didn't look for reviews before deciding on a largely Catholic story, but, now that you mention it, I'll look up reviews for Lord of the World. It's very different from my book, but avoids almost all violence and suffering. Basically, it skipped most of what's in Revelation. Fortunately, as I found in my research of Revelation study guides, Catholicism leaves it very open to interpretation, so I have a lot of room to maneuver.
EDIT: The reviews for Lord of the World were all over the place. Some loved it. Others thought it was awful.
Although I'm more than happy to have Rambo fans read the story, I'm definitely not worried about attracting them. If I was, I wouldn't be worried about the violence, and I would strip the prayers.