I hit AA 1MM around age 30, but as most of us know that's not terribly difficult. Especially once you start paying attention to FF miles and reading forums like this one. I was 20 when I joined AAdvantage. I had flown many times before and heard FA's talk about the programs, but never gave it any thought. The think that pushed me to sign up was that I knew I'd have to do 2-3 roundtrips to London.

At the time, I was probably thinking "Cool! That'll be enough for a free ticket home to KC from Chicago!" The thought that an MCI-ORD R/T might not be an optimal use of 25k AA miles would have never entered my mind in a million years.

Counting all air travel I've ever done in my life, regardless of airline or whether I even knew about FF miles, using very rough estimates for some defunct carriers, I've done about 800-900k BIS cumulative. Around 300k on AA, 150k US, 105k UA, maybe 75k WN (although that's a really rough guess), and a bunch of <25k figures for all sorts of other airlines.