It has been five years since he smoked.他吸烟5年了？

It has been five years since he smoked. 正确的意思是：他不吸烟5年了。

这个句子中的since比较难理解，再多看几个例子：

It has been five years since he got married.（一个event，事件）

It has been five years since he became a soldier.（一个event，事件）

It has been five years since he first smoked.（一个event，事件）

It has been five years since he was a soldier.（was a soldier是一种状态，类似 he smoked）

特别注意状态和事件的不同。再来看since的意思。

It is not grammar but semantics. Since means after something and before now.

对比while、before、after和since：

"While he was a soldier" or "when he was a soldier" was between the beginning and the end of his being a soldier.
"Before he was a soldier" was before "when he was a soldier" and thus before the beginning of his being a soldier.
"After he was a soldier" was after "when he was a soldier" and thus after the end of his being a soldier.
"Since he was a soldier" is after he was a soldier and before now.

Therefore,

A - It is two months since Tom (last) smoked a cigarette = It is two months since Tom has smoked a cigarette. = He last smoked a cigarette two months ago = He hasn't smoked a cigarette for two months.
B - It is two years since he left the country. = He left the country two years ago.
C- It has been five years since he was a soldier = It has been five years since he has been a soldier = He was a soldier five years ago = He hasn't been a soldier for five years.
D- It has been five years since he smoked. (the same as A)
E- It has been five years since he got married (or he married his wife)= He got married (or He married his wife) five years ago.(the same as B)(He has been living with his wife since then - for five years now.)
It is the same as
- It is five years since he bought his house = He bought his house five years ago. (He has had his house for five years now.)

参考：

https://forum.wordreference.com/threads/it-has-been-five-years-since-he-smoked.2912800/