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?)

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.

In the first set of sentences, all of them convey the same meaning. The choice between using "I" or "me" depends on the grammatical role of the pronoun in the sentence.

1. "He is not my age."
This sentence is stating that the person referred to is not the same age as the speaker. Here, "my" is used to show possession or ownership.

2. "He is the same age as me."
This sentence is comparing the ages of the speaker and another person, stating that they are the same age. "Me" is the object pronoun used after the preposition "as."

3. "He is the same age as I."
This sentence is also comparing ages, but "I" is used to showcase the subjective pronoun.

4. "We are of an age."
This sentence indicates that the speaker and someone else are roughly the same age. "Of an age" means being in the same age group or category.

5. "We are of the same age."
This sentence has the same meaning as the previous one, but it uses the definite article "the" before "same age."

6. "He and I are of an age."
This sentence is similar to #4, but it specifically mentions "He and I," indicating that both individuals are of the same age.

7. "He and I are of the same age."
This sentence is the same as #5, but it mentions specific individuals, indicating that "He and I" share the same age.

8. "He is as old as I."
This sentence compares the ages of two individuals, indicating that they are equal. "I" is used as the subjective pronoun to match the formality of "as old as."

In the second set of sentences:

1. "What grade are you in?"
This question asks about the academic level the person is currently in. "First grade" is a common response, indicating the person's grade level.

2. "I am a first grader in middle school."
This statement specifies that the person is in the first grade within a middle school setting. It implies that the overall educational structure might be different compared to a standard elementary school.

Regarding #1 and #2, they do not have the same meaning. #1 refers to the grade level in a general sense. #2 specifies that the first grade is taking place within a middle school environment.

In the dialogues:

3. The expressions in #3 are correct and make for a natural conversation. Each sentence is appropriately constructed and conveys the intended meaning of the speaker's interests and personality traits.