在二十多岁时英语

in his twenties是固定句型,更多含有“二十几岁”的意思,换而言之从额21 到29都可以包含,所以是用众数twenties,如果指明是二十岁的话,用twenty years old会适合点,即when he was twenty years old 或when he was twenty

一般来说是 20s，但也有说美英 20's，英英用20sIn British usage, we do not use an apostrophe in pluralizing dates: This research was carried out in the 1970s. American usage, however, does put an apostrophe here: This research was carried out in the 1970's.其他来源的解释：The Chicago Manual of Style recommends no apostrophe is something like 1960s.Ages are the same kind of thing. Consequently, the better form is:This is her brother WILLIAM (30s), fat, poorly dressed.所以最好不加撇号（apostrophe ），用20s

in one's twenties

inone'stwenties