The question seems a little weird because you sometimes use words defining the period of life (childhood, youth) and sometimes the ones referring directly to the person (infant, adolescent). You can ask if this is right:
0~7 infant
8~13 child
14~19 adolescent
20~25 young
P.S. Not really sure about the answer, I think infant is used only for very young perople (<= 2?) and there are a lot of other words for young child, like "toddler", but I don't really know how much these are used in everyday language