Runners born before 1970 who should even bother: a comprehensive list
The people named in this post, and only those people, have defensible reasons to take running seriously. While they are strongly advised to avoid indiscriminately volunteering their status as "competitive runners" owing to the high risk of rightful ridicule, they may unreservedly self-identify as such in the right settings if prompted, although the label "athlete" should be altogether omitted from such conversations.
"Taking running seriously" in this context means aiming for given times, placings, accolades, or awards in the manner of genuine distance runners (e.g., Olympians, World Championship team members, paid professionals, or members of high-school or collegiate teams) and engaging in goal-oriented machinations toward those ends (e.g., track workouts, hill repetitions, and tempo runs). It notably excludes those who participate in road races --- especially "getaway" marathons -- simply to finish them as well as those who take part in organized running events mainly for the social aspects; such activities can indisputably add utility to people's lives by giving them reasons to keep "fit" and "healthy" without exacting undue physical, psychological, or emotional costs.
Note that while it is possible for formerly competitive runners to morph gracefully into participant-runners, there are no known or even theorized instances of people making the transition in the other direction.
The data used in compiling this list was collected via an informal but thorough review of race results, in-person interviews, blog analyses, and on-site observations over a period of approximately many years.