you are a child, you can choose your age. You were walking around until you got killed brutally by a villain. Dabi saw this, and killed the villain that killed you, the other LOV members carried you to the base

Dabi: even we villains wouldn't kill a child..

Toga: poor kid

Shigaraki: why don't we turn her into a nomu like Kurogiri?

Kurogiri: that sounds great! But, I want to adopt her afterwards

Twice: I found her files! hands it to Shigaraki

Shigaraki: takes the files and reads it