My first UA flight was in 1965 when we moved from NY to CA. Don't think Mileage Plus was up and running then. I did write UA, though, and they sent me a nice package of 8X10 photos of their aircraft, which I wish I had held onto (at least I have my Disneyland postcards still from back then).

I first found out about frequent flyer programs in 1991 (back when an XT computer was considered fast), when I was online with Compuserve, Prodigy, and Genie. All of those services hosted Easy Sabre, and Prodigy had a promotion with UA offering 5,000 miles if you signed up with Mileage Plus and booked a flight on line with Easy Sabre. Seemed to be the right promo, and it was the first program I signed up for. Since then, I've banked 839,000 UA status miles (though I'm on AA now).