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.

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