I choose an age that suits the protagonist.
This is my approach too. How does their age impact their capabilities, who they are and influence the way others react to them, and help shape their story?
I've only seriously written fantasy set in secondary worlds thus far, so one of the things that influences my choice of age is whether I envision it primarily as a coming of age type story, or if it's about someone who is making their way in the world with the rights and responsibilities as an adult, but still not at the peak of their status or career. Or if they're more of an established, mature person within their society.
I picked early to mid twenties for one novel, because I wanted the characters to be past the YA and coming of age plot point, but young enough to still be struggling to find their place in the world. I'm thinking of having a woman who is a bit older, maybe late thirties or early forties, for another novel I'm blocking out right now, because I need her to be in a position of authority within her society and to be very certain of who she is, but still young enough to be treated as feminine by her culture, which will incur both benefits and perks. This will lead to the catalyzing event being even more disruptive for her than if it had happened to a much younger or much older person.
As a reader, I don't really care whether the protagonist is a child, teen, younger adult, or older adult. It's about them being an interesting character and having their age work for the story.