摘要：He was a big man in his forties, smartly dressed in a suit and tie. （他是个四十来岁的大个子，穿着西装打着领带，很时髦。）[柯林斯高级学者辞典]

in your forties是“在你四十来岁时”,如

【例句1】He was a big man in his forties, smartly dressed in a suit and tie. （他是个四十来岁的大个子，穿着西装打着领带，很时髦。）[柯林斯高级学者辞典]

【例句2】She's probably in her early forties.（她大概四十出头。） [剑桥高级学者辞典]

【例句3】He was in his early forties, with bulky shoulders and still reasonably slim round the waist. （他四十出头，肩膀粗大，腰围仍然相当苗条。）[柯林斯英语词典]

【例句4】He spent several years in Paris in the late forties. （五十岁前他在巴黎呆了几年。） [朗文当代英语词典]

【例句5】The woman was probably in her mid forties. （这个女人大概四十五六岁）[朗文当代英语词典]

这里的forties是40,41,42,.......,49十个数字的汇总，是多个forty, 所以是forties。

数学上40和41合起来也是forties, 但对自然语言forties是从40到49的时段。

事件发生在时段里，所以用介词in。

若是“在四十岁时”，则用at age forty。这是因为谈到at age forty, 我们认知唤起的年龄候选集是{......38,39,40,41,42......}。40是候选集中的一个离散点，所以用介词at。

作为对比，in the spring和in March的介词是in。这大概是事件发生在春季和三月里。

二十来岁是twenties到九十来岁的nineties都是规则的复数，但是十来岁是teens，不是tens。

就当前，带定冠词的the forties也指1940年到1949年这个时段。再过20年，它就该指2040年到2049年这个时段了。为避免误解可用the 1940s和the 2040s。

the forties也指任意40到49之间的量，最典型的是温度，如

【例句6】The temperature was up in the high forties. （气温高达四十度。）[朗文当代英语词典]

【例句7】The temperature is expected to be in the forties tomorrow. （预计明天气温将在四十度左右。） [剑桥高级学者辞典]

来源：文博教育课堂