The pair last year were two kids from the Seam who'd never, not one day of their lives, had enough to eat. And when they did have food, table manners were surly the last thing on their minds."
-Katniss and Effie talking about the pair last year in The Hunger Games
The District 12 Female participated in The 73rd Hunger Games at an age of 12-18. She wasn't volunteered for or voulenteering for anyone, as Katniss mentions on how there hasn't been a volunteer for her district in a long long time. Being from the Seam, as Katniss mentions, she probably resembled a skeleton, just like the pair from 12 during The 66th Hunger Games.
73rd Hunger Games
Since Katniss knew that she was from the Seam, she probably knew her on a somewhat personal way. However, her manners at the table were more worrying than anything. Effie mentions that she lost her appetite since she ate like a savage, since she never had a decent meal in her life. This angers her, since Effie doesn't understand how poor their district is.
During the start of the Games, she placed on a rank somewhat better than what a tribute from her district would get; as they aren't good fighters at all, and are very malnourished. She placed 14th by getting murdered by the District 4 Female, a career from the Games that we can assume was aggressive, (as careers tend to be). Since tributes from 12 don't like to pick fights in the Games (due to being the weakest ones when it comes to survival and fighting in the arena). District's usually have something to help benefit their tributes in the arena, like how the District 3 and 5 tributes are usually smart, or how the careers are obviously very athletic, and how District's 9, 11, 7, etc. give their children work at a young age, buffing them up. However, District 12 has no benefits whatsoever in the Games, as they can't go mining until they're an adult. And as we all know, its not a 100% chance that they'll reach the age of 18 when being reaped.