The Phenomenon Wherein Fictional Characters Don’t Age Relative To The Real World Is Called?
Answer: Floating Timeline
Someone who started reading the Batman comics as a child at the time the original comics were published, found Bruce Wayne as a young and ambitious man on the pages of those comics. If they ended up lifelong Batman fans, they found Bruce remained, more or less, perpetually existing in a state of age approximately 25-30 years old, or so. In fact, there’s a good chance that a child who first encountered Batman in Detective Comics #27 (published in May of 1939) may not even be alive today, yet, story arcs here or there aside, Bruce Wayne hasn’t aged a day.
This phenomenon, a sort of character immortality, is employed widely in literary fiction, comic books, and animation, as it allows for the franchise to persist over time and for the characters to appear in new story lines for generation after generation of readers.