Not a Million Miler but I did log in my United account today and got a pop-up thanking me for my 40 year anniversary in the MileagePlus program. Mostly amazed they've been able to track it this long.
Living in Hawaii in the 1980s, United was the dominant carrier to the mainland. At age 8 or so my dad signed us up or this "new frequent flier thingy that would give us free flights in the future". Although I'm still kinda pissed that they switched the MP IDs years ago since mine used to start with a like 4-5 zeros!