1、An infant (from the Latin word infans, meaning "unable to speak" or "speechless") is the very young offspring of a human or other mammal. the term infant is typically applied to young children between the ages of 1 month and 12 months;however, definitions vary between birth and 3 years of age.
2、A newborn is an infant who is only hours, days, or up to a few weeks old
3、When a human child learns to walk, the term toddler may be used instead
4、When applied to humans, the term is usually considered synonymous with baby, but the latter is commonly applied to the young of any animal.
總結一下：baby適用於所有動物，infant特指人類小孩，開始學走路的小孩叫toddler.
infant指1-12個月的小孩，有時也指出生到3歲的小孩。newborn指幾個小時或者幾天或者幾周大的nfant。醫學上認為newborn是出生28天之內的小孩。
age(baby) > age(toddler) > age(infant) > age(newborn)