Ira doesn't remember when she first wanted to die. It's there as a background fact in her earliest memories, along with the fact that it's Bad and Wrong that she pretends to be a girl all the time. (She keeps pretending anyways, in the privacy of her own mind.)
She's eight, in her earliest memories (...her memory, like the rest of her, was never very good at anything) and she already knows that if she steals her sister's clothes or tells anyone that she wishes she was never born, she'll get in trouble, and getting in trouble will hurt, sometimes a lot. And she's already learned to never ever tell people what she wants, how she feels. It's the obvious thing to do.