March 2002, so 19 years
a family member that was living with us at the time got me started off watching old reruns of the kanto and orange islands anime on cartoon network. it used to come on around 8pm. at this time i only knew the first 151 pokemon, togepi, and marill
then i remember seeing the entei and unown movie and realizing there were even more pokemon. i didn't know because the cartoon network reruns never played any of the johto anime. but there was a short that played before the movie that had pichu, houndour, igglybuff, and some others. somehow i had gotten a GSC handbook with the pokedex in the back so i was able to learn about almost all the pokemon i wasn't familiar with, except for igglybuff. i guess a page was missing from the book, but it took me the longest time to figure out what that thing was lol
then i got my first pokemon game in 2003 with Ruby. and i started following the anime in real-time with the episode with Larvitar and Unown