Bit of a meta one: are you

the accepted answer on Stack Exchange?

I've been here before I became me

The answer was here before becoming an accepted answer.

I'm wise and bold, you're about to see

Not sure about this.

Carly Simon, it's not my sin

I looked up Carly Simon and apparently she's a singer. Googling "Carly Simon sin" led me to this song - relevant?

I'm the one who'll have the green

The green tickmark. (This line was the giveaway, if this answer is correct.)

All of you, who are reading this
Will know my name if you persist

If you read the riddle, and then scroll down and read the accepted answer, you'll know the solution.

(If this all seems vague, don't scorn I'll explain it when I'm born)

The accepted answer, when it comes, should explain the riddle.

Bit of a meta one: are you

the correct/accepted answer on Stack Exchange?

I've been here before I became me

The answer was here before becoming an accepted answer.

I'm wise and bold, you're about to see

Not sure about this.

Carly Simon, it's not my sin

I looked up Carly Simon and apparently she's a singer. Googling "Carly Simon sin" led me to this song - relevant?

I'm the one who'll have the green

The green tickmark. (This line was the giveaway, if this answer is correct.)

All of you, who are reading this
Will know my name if you persist

If you read the riddle, and then scroll down and read the accepted answer, you'll know the solution.

(If this all seems vague, don't scorn 
I'll explain it when I'm born)

The accepted answer, when it comes, should explain the solution.