I can't say exactly when, I think it was around 1999, my cousin gave me Pokemon Yellow to borrow. Immediately hooked within 5 minutes! I was only 4 at the time, it was such a memorable game though... Maybe because I had good memories with it... :')
First started playing in the 3rd generation in 2003. I got Ruby for my 9th birthday about a month after it came out, and was hooked from there. I had been watching the series though on TV since 2000.
When I was in 8th grade I kind of fell out of love with Pokémon. I didn't really play again until HG/SS came out in 2010 and I got SS. I stopped playing that too after awhile. I didn't touch Pokémon again until Black/White came out. Soon after I stopped playing it as well, and recently I've been enticed back into my favorite childhood series now that the next generation is coming out. So 2003-2008 played nothing but Pokémon, came back in 2010 for a while and again in 2011, and now in 2013 I am playing again. Its been a bumpy ride but I just can't let go of Pokémon. I'll probably play it off and on for a long time to come.
Unlike pretty much every person in this thread, my first generation of Pokemon was actually the fourth in 2008! I had watched the anime a lot when I was a kid and I absolutely loved it (when I was younger, I always wanted a Pikachu because I considered it a legend amongst the ranks of Mewtwo / Kyogre / Mew etc.). I had watched my sister play Emerald for a while and then I decided I wanted to give the games a go as well, so for my 7th birthday I got Platinum! Yeah I'm insanely young compared to everyone else. Since then I've played Emerald and almost all of Crystal, I glitched up Yellow, played Heartgold, restarted Platinum 6082 times and I've also played Black 2. I still watch the anime but not as much. I also have a lot of Pokemon figurines from my trip to China and a few Trading Cards (that I don't really use unfortunately). So I've been playing around five years to answer the question? Despite that being nowhere near the 10 or 11 years most people have racked up, Pokemon still means a lot to me. :)