I'm 16, and I think I'm an oddity in this thread in that my first GT game was GT4, and I didn't start playing until under two years ago.
I did play racing games for quite some time when I was a lot younger, mostly NFS on the PC (II SE, III HP, Porsche Unleashed, etc.). Those weren't the most realistic games, of course, but it did introduce me to the more realistic aspect of racing games. I was introduced to GT4 in a very strange way: my sister decided she wanted to play Dance Dance Revolution at home, which required having a PS2, and after I briefly joined a couple of my (more distant) friends in playing GT4 (very poorly!) at their house, my dad surprised me by buying GT4 for me at the same time as DDR, even though I didn't request it.
I only played in Arcade Mode for quite some time until I figured out what was going on in GT Mode. I blitzed my way through in typical noob fashion, stopping at some high completion percentage after getting stuck on the Speedster and 206 races and Missions 23 and 34, at which point I played Burnout for a while. I reached 100% when I got back to GT4 and joined GTP to figure out how to beat those races. Then I got into OLR competitions, 200 A-Spec point racing, etc., and am still playing a lot. It helps that I don't have a PS3 for GT5P, I guess, although I would sure like to! But for now, I love GT4; despite its flaws, it's still an amazingly fun game.