Childhood isn't always necessary. It depends what you mean by biography. These days, what people call a picture book "biography" can also be a story about a particular accomplishment or event--like, I wrote one about something that happened to Benjamin Franklin when he was in France, and another about a pair of suffragists who drove around the U.S., and in both cases there was no reason to talk about their childhoods, even in the extensive back matter.*
Other times, a personality trait is the focus, and then it's typical to start in childhood and see how this kid always loved music or always had a can-do spirit or whatever. It just depends what angle you decide to take, and of course, what angle you decide to take depends on what material you find!
*(Oops, I just checked and I did actually talk about something in Franklin's childhood in the back matter. It wasn't really necessary, though.)