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.
Then, we meet a little girl named Lilac Laurel-Ember in the 84th Hunger Games. She is just 12 when she is reaped, and somehow manages to win.
The next year, it is time for the reapings for the 85th Hunger Games. And, Laurel's twin, Delilah, is reaped. Lilac was born at 11:57 PM the day before the reapings, and Delilah was born at 12:21 AM the day after. So, Delilah is technically 12 when she is reaped.
In the middle of those Games, Lilac is called to the Capitol for a special assignment. You can read about it, and the result of it, in my story "Rue Returns". What I can say is that Rue, Lilac, and Delilah are all mentors, too. Lilac and Delilah are mentors for 12, but Rue can be a mentor for either.
Now, Katniss is 42 and married to Peeta, and Prim is 38 and engaged to Damien. Katniss and Prim live next door to one another. Delilah and Lilac live in the next two houses. On reaping day, Lilac is 28 and Delilah is 27. Rue is 28, like Lilac, but is in 11 for reaping day (again, read "Rue Returns"for an explanation) and will be in attendance in 12 as a TV screen.
Haymitch is 67 and getting very old. He rarely mentors, and is only mentoring this year because he has to.
This should clear everything up for anyone who wants to submit a District 12 Tribute...