It is old enough for the boy to go to school. 这句英语的语法哪有错？？

正确的应该是： The boy is old enough to go to school.

应该是The boy is old enough to go to school
enough for 是指足够的，是量上边
例如
1. 足够的,充足的 We have enough chairs for everyone. 我们有足够的椅子让大家坐。 I was fool enough to trust him. 我真傻,竟会信赖他。 n. 1. 足够,充分[+tov][(+for)] We earned enough to buy a new house. 我们挣的钱足够买一幢新房子。 I've had enough, thank you. 我够了,谢谢。 ad. 1. 足够地;充分地[+tov] They cannot walk fast enough. 他们走得不够快。 I'm old enough to handle my own affairs. 我已经长大了,可以处理我自己的事情。

错误在于it is这个句型的使用。it是个代词，在这里它能指代什么呢？

英语，怎么简单怎么说，不用研究太多句型。
The boy is old enough to go to school.
这孩子够大了，可以上学了。

yes you are right
同意楼上的,但还要以这样说:
that boy is old enough to go to school