在他这个年纪英语

it depends...我的理解是 你想问 如果翻译：如果你是我（如果你在我这个年纪），你会怎么处理这个事情If you were me(if you were at my age),how would you deal with this(whatever this is...)?

When I was at this age 在我这个年纪的时候

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

不可以用in his thirty years old也不等于when he was thirty years oldin his thirties意思应该是在他三十多岁的时候

in one's age she can do something by hershef 在她这个年纪 她可以做一些事靠她自己