Until your laptimes are within .2 of each other for the whole race, I don't think the age of your kart matters much. Buy a used but solid cheapie, practice and get consistent, and then focus on whatever's slowing you down the most and work your way up (down?) the ladder from there. I'd also try to pick a class that has decent numbers so you can learn to navigate traffic, and a class that has good engine tech and stable rules so that the engine equation is minimized. Or if you want to bite off more than you can chew, know you're always legal, and never have to worry about tech, and also learn how to drive with both feet for a win instead of draft and bash, get into RWYB or Unlimited All-Stars (UAS) racing.
(*pokes bear.*) I'd say anything newer than 2000 with an adjustable front end is a good starter learning tool, assuming it's not cracked or bent.