1980
"Please God -- I'm 13 1/2... I can't be dying!"
This is from Kitty Pryde’s first appearance. 13 ½ and just finding out about her mutant power. This is shortly followed by another reference just before she officially joins the team.

"She's 13 1/2, cute, bright, spunky -- and she walks through walls."

1981
"Phasing her molecules through the wall, the thirteen year old X-man scampers to safety..."
Instead of gaining half a year, she lost one. Or, more likely, it was just a case of not bothering to add the “and a half” to it.

1982
"Suddenly I hated been fourteen -- a kid."
Kitty is finally allowed to turn fourteen.

1983
"Later still, in Kitty's cabin -- she's the youngest X-man, 14 years old -- a birthday having passed while she was in space, without her knowing --"
Except she’s magically de-aged back to 13. Now she’s fourteen.

"Happy Birthday Kitty" (From the decorations of her party)
See? There’s cake. As soon as there’s cake involved the age becomes holy writ. If that's not the way it works in Marvel, it should be. Because cake. There was also Obnoxio the Clown, which would make three times total Kitty's fourteenth birthday is referenced.

1984
"The mansion has been Kitty Pryde's home for nearly two years."
Okay, close to two years plus thirteen in a half equals fifteen pretty much any way you slice it, right?

"She's 14, Peter."
No, she’s back to fourteen again in the very same issue.

"<Police are seeking a young woman, a foreigner, aged approximately 15 years, for an attempted robbery last evening of a Matsushino Bank branch...>"
And then fifteen again, or 'approximately.' But then...

1985
"She's still Kitty here -- a girl, barely 15 -- but in the future, when I knew her, she was twice my age, part surrogate mother, mostly best friend."
"You're positively ancient... for someone a little past fifteen!"
"--how do you think assaulting a 15-year old girl is going to look on your records?"
Fifteen, certified three times.

1987
"Her name's Kitty Pryde -- also called Shadowcat -- she's barely fifteen."
"I am fourteen, going on fifteen, old as I'm gonna get..."
No! She goes back a year again!

1989
"If we're a match, she should be... barely fifteen."
Forward a year!

1990
"I confess it was a long time ago - positively the dark ages -- but I don't remember reacting so when I turned fifteen."
Back a year! And there’s cake! Everybody reset your clocks. Kitty’s age now on a whim-driven pendulum.

1992
"...here's Kitty Pryde -- Shadowcat if you prefer... can't be older than, what, sixteen?! Max."
About this time her age became a little less defined and was described more through conjecture. Just because he’s suggesting this is her age doesn’t mean we can bank on it. After all, there’s no cake.

"No -- actually it looks more like Marvel Girl, her identity when she was my age!"
Okay, same age as Jean, or somewhere in that range. All we can really get from that is she's not younger than Jean when she joined the X-Men.

1993
"..Kitty's old, like -- sixteen, seventeen?"
Is Jubilee being generous? Who knows?

Interestingly, over the next five years, her age is never stated. Not in a comic panel, at least. She does claim to be an adult, goes into a pub with only Douglock for company, and meets a SHIELD intern who is her age, but again, those are only clues to her age, not an outright number or suggestion of one. That doesn't come back until...

1998
"So tell me, little Kitty Shadowcat... since you're below the legal limit... whatever am I gonna do with you?"
More conjecture, this time from Sabretooth. It’s not really hard to guess which legal limit he’s talking about so… seventeen, then?

2000
"I'm barely sixteen, Peter."
Or not. This is still older than her last explicitly confirmed age, but in light of everything else that's happened since then, this seems like a stretch. Especially with what comes only two years later.

2002
"Five years ago, I lived in a different world, in a suburban Chicago town called Deerfield."
13 ½ plus five, so she’s eighteen now and has just been magically aged two and a half years.

2010
"21 Let's Party"