I've never really thought about the age of an author when I'm reading a book. To me, if a story has depth, warmth and meaning and is entertaining to read, the writer could be 18 or 80 and I'd enjoy it all the same. Unless the book is written in a particularly immature style, I would say there shouldn't be much to give away an author's age, really. It's about the words and the author's own personality coming through in the story.
Although from my own point of view, it intrigues me with children's writers, whether there's ever a difference between young writers of children's books and older writers. I've yet to notice it, but I always say writing for children means you kind of have to have a younger mindset anyway, whatever your actual age!