These are all from when I was about nine years old--
--Quilava evolving into Typhlosion on Route 12 or 13 (i.e. before the 4th gym), me having fought wild Pokemon and caught random whatever for...must've been hours, spread out over weeks, because I couldn't figure out that I needed the Squirtbottle from the floral shop to get past the funny Sudowoodo tree--I'd also never talked to Whitney after beating her to collect the Plain badge. Looking it up on Gamefaqs (through Verizon's search engine) is one of my earliest memories of using the internet.
--Beating Clair's Kingdra in extremely close fashion--before I'd figured out that playing with stat increases/decreases could've gotten me the win much easier. Watching it Rest off all the damage Typhlosion had inflicted in front of me was quite intense.
--Catching a shiny Rattata in the Tin Tower.
--Reaching Lt. Surge for the first time and hearing the Kanto gym leader music. I don't think it could've been more epic.
--Beating Red barely with Typhlosion and Lugia both at about level 73 or so and everything else in the 30s iirc. Snorlax was my greatest foe with Rest and Amnesia, and Blastoise--Lugia handled Espeon well. It was a real cathartic moment, I'd been playing that game for months at that point. Kept re-playing it through for the next two or three years, I was hooked.
Oh, and about a month ago, as I was sitting in my dining hall in the second semester of my freshman year of college at around noon, it occurred to me in a flash that Sudowoodo's name made perfect sense-- "psuedo-wood-o," i.e. fake tree. Oh the shock of that moment.