since I lived here; since I have lived here. 的区别? 从语法上看, 为啥会有这样的区别?

1. 一些典型的句子 (这些句子可能与错,不一定都是对的)

since用于引导时间状语的情况
I paid half price since I was young.
I paid half price since I was younger than twelve.

since用于引导时间状语的情况
I have gone to the opera every year since I was young.
I have been reading Tolstoy since I was younger than you are now.

那么下面几句是否语法正确?翻译成汉语是什么意思?
It is three years since she was in our class. (是否应该改成 It is three years since she left our class. ?)
It is three years since she has been in our class. (是否应该改成 It is three years since she came in our class. ?)
It is three years since he lived here. (是否应该改成 It is three years since he left here. ? 这句的含义会不会是"我已经离开, 但从当初我开始在这里居住到现在,已经三年了" ?)
It is three years since he has lived here.(是否应该改成 It is three years since he began to live here. ?)
It's been YEARS since I have done this! (https://www.youtube.com/watch?v=v8n0jaMnmiM&pbjreload=101)

2. 在网上看到的一些相关解释.

没有明白下面截图中的两句话的含义, 也没有明白since I + did, since I + have done两个表达的区别.

3. 再看第二个网上资料.native自己解释也不尽相同.

(https://www.quora.com/Whats-the-difference-between-these-two-sentences-is-5-years-since-I-have-lived-here-and-is-5-years-since-I-lived-here)