Original translation: There is no time like the time when you and I were young.
I interpret this differently:
It is no longer a time (period) like when you and I were young.
Although I find the 今は to be in a strange position, if it were at the beginning of the sentence it would make it easier to translate. It may help contrasting old time and now, maybe?
今はもうない itself is a very natural phrase and 今は should be close to もうない. “It no longer exists” is the meaning.
あなたと私「とが」, 若かった「時」and 時代「が」are unnatural, though.
is the correct usage of particles. But I still feel this altered sentence robotic. My final answer is
あなたと私が若かった頃とは時代が変わってしまった。(Lit. = The current time has totally changed from that in your and my youths.)