most ppl i knew those days (and i knew a lot) where around 18-25 in the first 2 vanilla years. vanilla DEFINETLY was not played by a lot of ppl under 17-18 years. those ppl were an absolute minority these days. maybe later, when everyone and his mother heared that wow is THE game today. but in the first 2 years of vanilla that was an absolute minority.
and thats not surprising, bc in those days literrally no one played mmorpg games. there was a handful of ppl playing star wars galaxies, dark age of camelot, everquest and ultima online. most ppl playing wow those days were comin from diablo 2. all of them blizzard fan enthusiasts, already waitin for wow. blizzard games at those times had a high quality standard (not as today) and there was a big fan community around this philosophy, that played warcraft 1-3, starcraft 1, and diablo. those ppl just waited for that BIG new blizzard wow thing.
so, in short: in my exp nearly no one was under 17-18 (way more 23-25) in the first 1-2 years of vanilla.