He is not my age.

He is the same age as me.
He is the same age as I.
We are of an age.
We are of the same age.
He and I are of an age.
He and I are of the same age.
He is as old as I.

(Are they all the same? Do I have to use 'I' or 'me'?)

1. What gread are you in?
I'm in the first grade.
I am a first grader in middle school.

2. What year are you in?
I'm in the first year.
I am a first year in middle school.

(Are they correct? Does #1 and #2 have the same meaning? Are they the same?)

3. What are you inerested in?
I am interested in collecting coins.
How about you? I am interested in reading newspapers.
What is your personality like?
I am kind of reserved. What about you?
I am outgoing, but I am a little reserved sometimes.

(I made a short dialogue. Are the expressions in #3 correct?)

2 answers

He is not my age.
He is the same age as me.
He is the same age as I.
We are of an age.
We are of the same age.
He and I are of an age.
He and I are of the same age.
He is as old as I.

(Are they all the same? Do I have to use 'I' or 'me'?)
Use "I" not "me" - and the expression including "of" is not in everyday use. "We are the same age" is normal. How would you rephrase the others?

I am a first grader in middle school.
In the US, middle school usually starts with 6th or 7th grade, not 1st. Otherwise, these expressions are fine.

I am a first year student in middle school.

<b<Everything in #3 is correct. =)
He is not my age. (Would be more specific to indicate younger or older.)
He is the same age as me. (colloquial/informal)
He is the same age as I. ("am" is inferred.)
We are of an age. (not used)
We are of the same age. ("Of" not needed.)
He and I are of an age. (not used)
He and I are of the same age. ("Of" not needed.)
He is as old as I. ("am" is inferred.)

A "first grader" would be in the first year of elementary school.

The expressions in #3 are correct, but the listener is likely to be at a loss to respond to the question on personality. "Like" can be substituted for "am interested in."

I hope this helps. Thanks again for asking.