Both hubby and I can hear it -- 28 and 29 years old. Man, it is ear-piercing!
This sound was originally used as "kid-repellent" outside stories in Britain. Some smart kids turned it into a ring tone.

One 555-timer a few basic linear circuit components and a speaker, and you too can have a 17+kHz generator (though it may need amplification.)
Less than $15 at your local Radio Shack (or less than $5 if you go to a place that doesn't rip you off.)
Of course, my child repellent works well. I just tell them one of my two jokes that I know and they leave.

Maybe 7-11 ought to use that tone to replace the classical music that they used to pipe outside of their establishments to scatter loitering youth. As Bill Cosby used to call jazz music, "child-repellent". Or maybe I could learn to make that sound when children around here start bickering in my presence. Hmmmmm....

I'm 36 and I can REALLY hear it. Wow, is that obnoxious. Plus I bet it would drive a dog to distraction. (Or possibly call bats to do your evil bidding.)
I'm sick of the electronic rings in general. When I was out somewhere recently, someone's cell phone rang and it sounded the way real telephones used to sound, and I thought, "Boy, do I miss that!!!"

Sorry, the correct response to Jimmy's post is:
"What did you say?"

Those things hurt. After listening to the original plus some of the ones slacker above posted (is that a slackware reference?), my ears are ringing a bit themselves.
Want fun? Get a frequency generator and a speaker.
My ears are still pretty good. My wife, who was a regular, front-row, concert goer can't hear me when I talk to her in the car at a normal voice without working at it. She's several years younger than me. :) I think the increase in compression during mastering and on the radio plus the increased usage of mp3 players will make this only a passing fad. Sadly..

Mosquito sound files can be found here http://anti.mosquito.googlepages.com/download

I have the perfect solution.
Let your child's FIRST cell phone be a high school graduation present!
Until then, if they really need one (like on a trip), let them borrow one of the family phones.
If they abuse the privilege, let the last high-pitched sound their phone makes be the sound as it is being ceremoniously cut in half with a band saw (they should be present for this).

Wow, talk about tension you can cut with a knife! (referring to the link in the first comment)

I'm incapable of hearing what my husband says from another room, especially when I have water running at the sink in front of me or when he's not enunciating well ( ARGH! ;-) )
But I can hear that and I'm 33. Yes it sounds like a monitor or TV hissing, really high-pitched, and in one media player (VLC) I can hear some modulation as well. This is on cheap $20 computer speakers. With my headphones I do not hear it, maybe because they don't 'do' that frequency.
I don't hear it as a loud noise but for some reason it hurts my ears anyway, and I couldn't be exposed to it for 30 seconds without running away.

Do I feel old! I'm 46 and I can't hear it. Strange, because at normal frequency I have particularly accute hearing.

I'm 34, and I can hear it quite easily. It sounds like a TV screen or an old computer monitor (one with a low enough refresh rate so that you can hear the high-pitched sound it makes), only the ringtone is much louder and more annoying. Why on earth would anyone *want* to hear this noise? As a previous commenter said, just put your phone on vibrate or silent mode -- what's so hard about that? :)

I have a portable voice recorder that takes MP3s. I'm tempted to stick that file on it and go bug some younguns with its pint-size speaker.

I'm 21 and it makes me cover my ears in pain. Its like someone putting a drill in your ear.

I can feel it, and I can hear a squealing noise, but it doesn't at all sound like a phone.
I'm a grup! Aaaaghhh!

So some of us are grups and some of us....
It'd be horrible to miss a bunch of phonecalls just because your ears suddenly matured, though.

I hear it, I hear it!. It's already off but I can still hear it!
Darn, I wish I didn't turn up the volume.

Ok, I'm 31 and I can hear that. In fact, it kinda makes me nauseous.
Not good.

Just last night I was tucking in my 4-year old son, when he said, "Mom, the phone's ringing." From where we were in his room I could not hear it at all, but it was obvious to him. I had just read about this so I knew what was going on!

I have to add that I'm ultrasensitive to high-pitched or low-level noise. The hum of certain electric motors (particularly fans) actually cause me pain. Maybe that's why these old ears (51 year old this Friday) can hear the ringtone.

I can hear it -- it sounds just like any innocent background noise, rather like the buzz of a fluorescent lightbulb. I wouldn't normally notice it, but now that I'm "tuned in" to it, I probably would.

Why not just put their phones on vibrate, or silent. In my day, we had to make due without mosquito ringtones, and that's what we did.
Kids today...

So what are the kids doing with it?
Using it to receive cell phone calls and text messages in class without the teacher noticing.
Just let them try it! We teachers have the ability to hear sounds that no one else can hear.

But back to the "mosquito" post.
I downloaded the ringtone from here:
http://www.npr.org/templates/story/story.php?story...
Then I played it reasonably loud, and couldn't hear a thing, but three of my kids doing homework in other rooms shouted out in unison: "What's that noise?!" and came rushing in.
Now they want me to bluetooth it to their cell phones.

Well someone heard a ring tone today - and when they answered, they got a rather unexpected voice on the other end!
Jimmy Akin on James White's show:
http://www.aomin.org/dl15.ram
Starts at about 56:30 into the program.
Better warn Jerry Usher's call screener!