英语语法高手请进

错的。代词和相应的动词'与先行词不一致。
建议把someone改成复数的单词，比如说 people.
People's youths are the period of their lives during which they are children before they are fullly mature adults.
如果要用Someone做主语，句子就会变成
Someone's youth is the period of his/ her life during which he/ she is a child before he/ she is a fully mature adult. 但英语语法专家表示，这样的写作是比较低级的，因为显示出writer没有能力可以简洁的表达出他的想法。如果单用he或she 做代词，就会涉及到性别歧视，这在欧美国家是极为敏感的。

如果觉得回答的好，LZ就采用吧。^-^

我倒觉得此话正确，最好问外国人。我记得语法书说，“某人”someone, anyone 等不定代词，用人称代词指代时，多用复数形式 they, them, their，虽然有时也用单数形式 he, him, his。所以这句话我认为只有英国人、美国人等英语为母语国家的人才可以给出满意答案。

没错。回应someone 的代词，即可用he/him/his,也可用they/them/their.
例句：Someone is against the plan,is he?/aren't they?

someone做主语时为单数。
我觉得改成Someone's youth is the period of his life during which he is a child, before he is a fully mature adult.好一点。