Here is the backstory for these games...
Peeta was never reaped-but Prim was. Katniss volunteered for Prim, and instead of Peeta, a random, super-awkward boy was reaped and killed in the bloodbath. Most of the major plot events in the Hunger Games still happen, including rue's death and the mutts. Also, Katniss still wins the Hunger Games.
The card for the 75th Hunger Games read: "As a reminder to the rebels that even the youngest and the oldest played a role in the Dark Days, the only ages eligible for reaping are 19 and 11." Gale was reaped, and Katniss had to mentor him. Gale died in the final 10.
During those games, Peeta introduced himself to Katniss, because he wanted to ease her sorrow. They soon fell in love. Meanwhile, Prim fell in love with a boy named Damien O'Donnel. He was reaped for the 76th Hunger Games-and won.
Last year (the 84th Hunger Games) a little girl named Lilac was reaped. She had just turned 12 the day before the reaping, making her the youngest victor ever. She was 12 and a month, give or take a few days, when she won.
Now, Katniss is 26 and married to Peeta, and Prim is 22 and engaged to Damien. Katniss and Prim live next door to one another, with Haymitch on the other side. Katniss's mother lives with Prim.
Oh, and by the way, Katniss just had her 1st child a few months ago, so she is not mentoring this year.
This should clear everything up for anyone who wants to submit a District 12 Tribute...