I am not certain if a person can (according to the rules) become a MP member until they are 18. Does anyone know?
I got my first UA Mileage Plus card when I was ~6 in the 1990-91 timeframe (but unlike the over-achievers here
didn't do any serious traveling until much, much later)... back in an era where paper tickets and travel agents were de rigueur and I had less than a dozen segments per year, all intra-California.