Translation:If we had met when we were young, life would be very different.
How do we know who was young? Could it not just be the speaker i.e. "If we had met when I WAS young ....."?
The "while"/"-ken" construction uses the same subject as the verb UNLESS otherwise specified. In order to say what you are suggesting, the sentence would have to say "...ben gençken..."
It is possible to say "birbirimizle", actually it's the most correct one.
"If we had met with you when we were young, life would have been very different." Is it OK?
No, "to meet" and "to meet with" are different concepts and verbs in Turkish.
What about "If we had met you when we were young, life would be very different"?
In one example olurdu means 'would have been' but here olurdu translates to 'would be' - so which is it and why is it right in one excercise and wrong in the other?
If we met you when we were young, life would be so different cümlesi de kabul edilmeli
if we had met with you when we were young life would be very different
it is marked wrong just for WE, why?
Seninle is not be translated in answer. Why? Moreover shall we use birbirimizle yada bizimle for seninle?
"seninle" is not neceassrily needed here.
Eğer gençken karşılaşsaydık hayat çok farklı olurdu". ("If we had met when we were young, life would ve very different").
"Seninle"is needed if we were to write this sentence as:
"Eğer gençken seninle karşılşsaydım hayat çok farklı olurdu". ("If I had met you when I was young, life would be very different").