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.
总结一下：baby适用于所有动物，infant特指人类小孩，开始学走路的小孩叫toddler.
infant指1-12个月的小孩，有时也指出生到3岁的小孩。newborn指几个小时或者几天或者几周大的nfant。医学上认为newborn是出生28天之内的小孩。
age(baby) > age(toddler) > age(infant) > age(newborn)