for my peer group
I was no longer young for my peer group, nor did I walk around in gray sweatshirts with unlit cigarettes in my mouth.

What does the first sentence mean?
I was no longer young is enough to express the same idea.
Does the speaker compare his age with that of his peer group?

Question

for my peer group
I was no longer young for my peer group, nor did I walk around in gray sweatshirts with unlit cigarettes in my mouth.

What does the first sentence mean? 
I was no longer young is enough to express the same idea.
Does the speaker compare his age with that of  his peer group?

Answer

A peer group is a group of people that are the same in some way.
At school your peer group is those who are in the same year.
If, for example, you skipped a year of school by being identified as bright, then you would be "young FOR YOUR PEER GROUP".
.
The emphasis is on the comparison of the speaker/writer and their peer group.
.
So does it mean immature or physically younger, you can only tell from context.

Answer

Yes, I think he means that he USED TO be 'young' for his age - that is, act immaturely, but now he has reached the same maturity as those around him - his peers. :)

Answer

I think the speaker is comparing the differences between him and his former group of friends that he has grown distant from.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!