No actually you were the perfect age. I would say the target demographic for Pokemon is age 5-12. They make the show very friendly looking and such to appeal to that age group of kids. That's why Ash remains 10, they want to appeal to their target demographic. Though if you look at the ages of most people here, you see the range is mainly people between 14-25 and even people in their 30s and maybe 40s. This proves that Pokemon doesn't really have a target demographic. So no problem with you being 6 and finishing Pokemon Crystal. People were probably just found it unbelievable because of your age. People don't really expect much out of a six-year-old. Most people think they don't have a good enough attention spam to complete an entire game by themself. So it's just a bit surprising for them when you tell them that.
Yay, and we are 20. D: Nintendo or Game Freak thought that it was only for 5-12 after maybe the 2nd Generation. With all the people with varying ages coming to GameStop, TRU or everywhere there would be an event Pokemon to be distributed or Pokemon VGC, Nintendo/GF now knows that Pokemon appeals to teenagers above. It is a good game from the start, even if it is repetitive, and we all love the game.