An infant is a baby 0-2 usually an infant can't walk yet or just started to walk and then when the baby can walk it is now called a toddler is 2-3 and then 4-12 is childhood and 13 -19 is youth/adolescent which is the same thing... In other words a teenager. Hope this helps

It’s not really 100% clear cut but… Newborn: under 3 months. Infants 1 month - 1 year. Toddlers: 1 - 3 years preschooler:  3 – 5 years adolescent (teenager): 13–18 years Childhood: from toddlers to adolescence. But it can vary so that it doesn't include teenagers. Youth varies but the United Nations defines with as 15-24 years. Youth can also be used metaphorically. So if someone is 80 years old but acts really young, you can call them youthful. Technically, in USA, adulthood is 18 onwards. English is complicated ㅋㅋㅋ

Wow.. infant and adolescent were words referring to people :) indeed I didn't know it and just thought they were for the periods. Thanks anyways!! ^-^

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