How do you say: "I have known her since we were both 2 years old"?
"I have known her since we were both 2 years old."


I would translate this as simply:


  我们两岁就认识的  "We were just 2 when we got to know each other"


Sometimes the tricky part of translation is that different languages communicate the same information using different structures, or a structure that is natural in one language is very weird in the other. 

The word "since" can only be translated as 自从 in certain contexts. In sentences like this, it is more natural to switch around the word order and use 就. Here's another example:


  I've been here since 3 o'clock! Where are you?!
  我三点就到这里了！你在哪里啊？！


Hope that helps.