Five-month-old 可以用来当名词或形容词(形容一个人或一件事)。 好比说,"I have a five-month-old at home" (我家里有个五个月大的孩子),"This five-month-old baby girl is really cute" (这五个月大的女婴很可爱),"Don't eat the five-month-old meat" (不要吃放了五个月的肉)。
Five months old 纯粹是形容词,但只能用来形容主语/主词。好比说,"My daughter is five months old" (我女儿有五个月大),"I have a daughter who is five months old" (但不能说 five months old daughter), “At five months old, infants are usually ready to begin experimenting with solid foods" (五个月大的时候,婴儿一般可以开始适当的试吃干的食品)。
参考资料:我7岁移民,从小在美国长大,现在是 UC Berkeley 的毕业生。