I don’t see a problem here. Read what you want, regardless of your age.
And yes, as the article states, there is plenty of fiction technically classified as “YA” that can appropriately be called “crossover” fiction.
By the same token, you might fairly argue that Clive Cussler’s novels (categorized as adult fiction) are really geared toward the reading level and tastes of a fifteen-year-old boy.
No matter, plenty of middle-aged adults (myself included) enjoy Cussler’s work.