This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
It's hard to believe, but this is one of the questions that keeps X-fans
arguing over drinks at all hours of the night. This is because Kitty,
who joined the X-Men in the (our-time) 1970s when she was 13 1/2, was
still a teenager in the 1990s. She was extremely popular with the young
crowd just getting into the Claremont/Byrne run of the X-Men, since she
was roughly their age and was a witty, smart woman that you could either
agree with, or like to date, as the tastes may be. Claremont used to
keep track of her birthdays rather closely, but those eventually fell by
the wayside. She had a birthday in space (14th) during the 160s of UXM,
and she had a birthday in England (15th) in issue #26 of Excalibur.
Kitty made a reference to being old enough to drink in Excalibur #91,
which most people read to say that she was 16 to 18, based on Scottish
law. Warren Ellis, former writer for Excalibur, was quoted as saying he
thought she was 18, which was a proper age for a young woman having lots
of snogging with an older man.
In UXM #379, however, Kitty remarked to Colossus that she was "barely
sixteen." Many fans were upset because it seemed that the writers were
trying to retcon Kitty's likely sexual relationship with Pete Wisdom by
making her underage. The issue was credited to Alan Davis, but the style
of dialogue strongly suggested that Claremont (who was returning to the
titles) had ghostwritten it.
At the 2000 Chicago Wizard World convention, someone asked Claremont to
clarify Kitty's age, and he said: "She is what she is. She is below the
legal age of drinking and always has been." That said, she could be
18 but not 21, especially if Claremont was just referring to the
American drinking age and not the British one (which is eighteen in
Scotland). This is supported by X-Men Unlimited #36, which was written
by Claremont. In that issue, Kitty states that she left Deerfield five
years ago, she is enrolled at the University of Chicago, and she is
working at a bar in Hyde Park. Illinois law requires servers of liquor
to be at least eighteen years old (though one still can't drink until
age 21); additionally, Kitty was 13 1/2 when she left Deerfield. She's
got to be at least 18 or 19 years old by that reckoning.
Then there's Jubilee, the other young teenager. Jubilee was originally
about 15 when she leaped from the mall to Australia via Gateway's portal
(UXM #244), but Scott Lobdell saw it otherwise. In the Generation X
collector's preview, he decreed that Jubilee was 13 1/2, and as her
writer, his word was law on the subject. Near the end of the run of
Generation X, in issues #65-69, she says she's 16, so she's grown older.
As for the rest? There are no easy answers to that question. Neither the
writers nor Marvel Time are fairly consistent. There are some fixed
dates: Nightcrawler turned 21 in UXM Annual #4, Rogue was 17 when she
joined, Hank angsted about turning 30 in X-Men #20, and in X-Men #51
Scott said he was "twenty-fi--". More such dates exist. However, even
these examples demonstrate a problem: Hank is not five years older than
Scott. The original X-Men were under 20 when they started, and Bobby was
the youngest at 16.
Some of the former New Mutants also had ages stated on-panel. In Graphic
Novel #4, Juliana Sandoval mentions that she and Roberto are 14. In New
Mutants #2, the ages of four New Mutants show up on their computers:
Rahne 14, Bobby 13, Sam 16, and Xi'an 19. (For context, in NM #3 Banshee
remarks that "t'day's Kitty Pryde's [14th] birthday.") A letters page
soon afterwards corrected the ages, showing 'Berto as 14 and Rahne as
13, the youngest New Mutant. This helps, but only somewhat, as who knows
how many Marvel-time years have passed since then?
There is also the problem of real-world events; Forge fought in Vietnam,
and Xavier fought in Korea. Unless, as Paul O'Brien suggests, the wars
were fought more recently in the Marvel Universe, the numbers simply
don't add up.