I had to do a bit of research before I could respond to this, and it turns out creaky voice is kind of a fraught issue that isn't completely understood. But I'll do my best!
First of all, what exactly is creaky voice (aka vocal fry)? This video gives an exaggerated example so you can hear it clearly--you'll probably recognize it once you hear it (but you can ignore the rest of her rant):
To produce creaky voice, you compress your vocal cords, which makes them slacken and vibrate slowly and irregularly. This is why creaky voice sounds low-pitched and, well, creaky. Here's a visualization of my voice saying "alphabet":
The dark vertical bars each represent a single vibration of my vocal cords. In the first "a" sound, the bars are regularly spaced and close together; this is normal voicing. During the "e" sound, you can see that the bars are more spread out and irregularly spaced--this is creaky voice. (The blue line shows my pitch; the program I used to make this image gets easily confused about the pitch of creaky vowels, which is why I apparently have no pitch there.)
It turns out that everyone uses creaky voice sometimes, especially at the end of a sentence. It's often correlated with low pitch (also frequent at the end of a sentence). I didn't really try to say "alphabet" with creaky voice; it just happened naturally, and I'm not a particularly creaky talker. And this is true of people of all ages, not just young people.
Researchers have also discovered that creaky voice in English and other languages can have a paralinguistic function: that is, it doesn't directly affect the meaning of what you say, but it does aid in communication in some way. For example, it can be used to signal the end of your "turn" in a conversation. In Australian English, creak can signify that you have low solidarity with the person you're talking to. And if you say "yeah" with creaky voice during a conversation, it can mean that you're allowing the other person to take their turn talking, or that you want them to change the subject.
So creaky voice is not restricted to teenage girls. But it hasn't been fully established who actually uses it the most. (For a longer discussion, see this post at Language Log.) This is what I've been able to glean:
- Creaky voice has to some extent been associated with men in the past (it sounds kind of tough and growly, I guess).
- There is some evidence that extensive creaky voice is used by young women on the West Coast, and possibly spread from there. But older women might use it more than men too.
- Creaky voice is perceived by many young people as informal, urban and upwardly-mobile.
- Unexpectedly, NPR interviewers apparently use a lot of creaky voice! Maybe it makes them sound calmer and more disinterested?
So, bottom line, creaky voice has been around forever and has several conversational and social functions. So try not to let it bother you!
P.S. There are some languages (including several native Mexican languages) that use creak contrastively; that is, a word with a creaky vowel will have a different meaning than the same word with a normal vowel. Creak is a pretty versatile thing!