▲关于这个问题，你先要明白 one-year-old 这类词既可用作形容词，也可用作名词。用作形容词时，只用于名词前作定语，不能用作表语；用作名词时，它是可数名词，根据需要既可连用冠词或其他限定词，也可以用复数形式。

▲你的句子中的 one-year-old 为名词，相当于 a baby of one year old，one-year-olds 为其复数，相当于 babies of one year old。句意：Their work shows that one-year-olds are much more likely to have allergies if either parent smokes. 他们的研究显示，如果父母任何一方吸烟，1岁的婴儿更有可能患过敏。

▲举例

1. 类似 one-year-old 这样的词用作形容词

Her two-year-old son toddled into the room. 她的两岁的儿子摇摇摆摆地走进屋里。

The 15-year-old schoolgirl is being groomed for stardom. 这个15岁的女学生在接受演员培训。

Even a six-year-old can have an accident at night sometimes. 即使是六岁大的孩子，晚上有时也可能会尿床。

My four-year-old daughter is always trying to imitate her older sister. 我四岁的女儿总想模仿她姐姐的言行。

2. 类似 one-year-old 这样的词用作名词

Eighteen-year-olds qualify to vote. 年满十八岁者有选举权。

The two-year-old sucked his thumb. 这个两岁的孩子吮吸他的大拇指。

He reads well for a six-year-old. 就六岁的孩子而言，他的阅读能力不错。

Our three-year-old loves her red tricycle. 我们三岁的女儿爱骑她的红色三轮车。

Most seven-year-olds are grounded in the basics of reading and writing. 大多数七岁的儿童都要接受读写的基本训练。