在他五十几岁时用英文怎么写

这个字用英文写in his fifties。
in加one's加整十数的基数词的复数形式，表示“几十多岁”的意思，如：
He is in his fifties.
In his sixties, he rode his white bike all over the country.
The boy was in his teens.
He is not yet out of his teens.
希望我能帮助你解疑释惑。