"Fifteen" is a song written about Taylor's freshman year at Hendersonville High, more specifically about her best friend Abigail. Taylor explained to iheartradio on an article that came out on April 9th:

"I wrote it about the journey that by best friend, Abigail, who, you know, we're still tight. We met in freshmen year of high school (...). And so I wanted to tell the story of being 15, kind of through the lens of a friendship. And, you see what these two people go through together friends and on their own. It still gets me, this song, because those were all real insecurities of like walking in your freshman year and thinking like, where am I going to fit in, in this massive ocean of people? Like, where's my locker? How do I fit in here? You're like, who am I going to be? That's a question that we ask ourselves a lot through true course of life, but I think never more than when we're 15."

In a piece for her old label's website in 2010, Taylor also revealed that "Abigail gave everything she had to a boy who changed his mind" was the first line she wrote.

Lyrics-

You take a deep breath and you walk through the doors
It's the mornin' of your very first day
You say hi to your friends you ain't seen in a while
Try and stay out of everybody's way
It's your freshman year and your gonna be here
For the next four years in this town
Hopin' one of those senior boys will wink at you and say
"You know, I haven't seen you around before"

'Cause when you're fifteen and somebody tells you they love you
You're gonna believe them
And when you're fifteen, feelin' like there's nothin' to figure out
Well, count to ten, take it in
This is life before you know who you're gonna be
Fifteen

You sit in class next to a redhead named Abigail
And soon enough, you're best friends
Laughin' at the other girls who think they're so cool
We'll be outta here as soon as we can
And then you're on your very first date and he's got a car
And you're feelin' like flyin'
And your mama's waitin' up and you're thinkin' he's the one
And you're dancin' 'round your room when the night ends
When the night ends

'Cause when you're fifteen and somebody tells you they love you
You're gonna believe them
And when your fifteen and your first kiss
Makes your head spin 'round
But in life you'll do things greater than
Dating the boy on the football team
But I didn't know it at fifteen

When all you wanted was to be wanted
Wish you could go back and tell yourself what you know now
Back then, I swore I was gonna marry him someday
But I realized some bigger dreams of mine
And Abigail gave everything she had
To a boy who changed his mind
And we both cried

'Cause when you're fifteen and somebody tells you they love you
You're gonna believe them
And when you're fifteen, don't forget to look before you fall
I've found time can heal most anything
And you just might find who you're supposed to be
I didn't know who I was supposed to be
At fifteen

La-la-la, la-la-la, la-la-la-la
La-la-la, la-la-la, la-la-la-la
La-la-la, la-la-la

Your very first day
Take a deep breath, girl
Take a deep breath as you walk through the doors