Left town with a dime to my name
Said I'm done with all my fake friends
Self-righteous pawns in a losing game
Got my band and a light that won't go out
Been a burning since the day I was born
So I cried just little then I dried my eyes
'Cause I'm not a little girl no more
And so if I have to I'm gonna leave you behind
Some of us have to grow up sometimes
And so if I have to I'm gonna leave you behind
Something I will never understand
Aren't you tired of always being mad at the world
Won't you just admit you don't care
And lady, I don't want your pity
So don't feel sad for me
I’ve got a love I would die for and a song to sing
Maybe we're both just living out our dream
And so if I have to I'm gonna leave you behind
Some of us have to grow up sometimes
And so if I have to I'm gonna leave you behind
Me and reality, the light and the dark
And we live alone
And two different worlds
Me in a fantasy
You in your memories
We get along, for the most part!
We get along, yeah, for the most part!
We get along, for the most part!
We get along
We get along
We get along
And so if I have to I'm gonna leave you behind (leave you behind)
Some of us have to grow up sometimes
And so if I have to I'm gonna leave you behind (leave you behind)
Some of us have to grow up sometimes
And so if I have to I'm gonna leave you behind (leave you behind)
Some of us have to grow up sometimes
And so if I have to I'm gonna leave you behind

To me it sounds like Hayley is trying to move on again like Fast In My Car and being new and starting all over again. Making new goals, making new friends, etc.

"Got my bedroom right then woke you out / Been a burden since the day I was born"
Jesus Christ, it's "got my band a light that won't go out / been burning since the day I was born" who came up with these lyrics?
Also, the part that's supposed to go "And baby, I don't want your pity / So don't feel sad for me / I got a love I would die for and a song to sing / Maybe we're both just living out our dream" is totally directed towards Sherri Dupree. Sherri wrote the song "Sad" about how sorry she feels for Hayley for dating her cheating ex-husband, Chad. Damn.

These lyrics are killing me, They are very wrong.
Anyway Grow Up is one of my favorite's from the new album Paramore. I think its because I can relate to it so much right now. Im at the point in my life where im letting go of certain negative people/friends in order for me to get on with my life and to well grow up.
I truly think this is what this song is about. There comes a point in everybodys life where they have to let go of certain people even if they don't want to, but they know its really for the best, for them to move on.
Also I agree with @afflictionate with the whole "And baby, I don't want your pity / So don't feel sad for me / I got a love I would die for and a song to sing / Maybe we're both just living out our dream" being about Sherri Dupree. But I think instead of "Baby" it says "Lady" thats what it says in the booklet that comes with the album, and lady does make more sense then baby.
But yes it makes so much sense because apparently Eisley's song sad is supposed to be about feeling sorry for Hayley, and then Sherri did write that lengthy blog about the whole drama.
Hopefully this isn't too confusing or too long.

People, use the edit feature to correct lyrics. :)

i think this song is totally directed to the 2 band members that left

Aghhh, just looking at the incorrect lyrics really bugs me. :/ Grow Up is the trio sticking together and not letting anything (or anyone) stand in their way. Leaving the past in the past.