For me it was about age 14, when my interest in art blossomed. And I wish I had known to take my desires at that age seriously, rather than assuming I’d outgrow them. Later I switched this and tried that, and wasted many years looking for a more “acceptable” way of earning a living. Eventually I was back right where I started.
Anybody else?
39 here as well, and I’ll let you know when I decide to concentrate on something. 
17, when I discovered that programming our primitive high school computer in machine language was both fun and something I did very well. It’s never stopped being fun.
Well, this makes me feel a LOT better. 37 and I still don’t know what I want to be when I grow up.
I was 40. Two years on, and the calling is still there.
We should start an “inadequate career” club of some sort.
I dont know yet what my calling is. But I hope to god I figure it out before I retire!
I guess I’ve known since I was seven or so. I’m now 28 and have still to get going with it.
34 and still more or less winging it. Not doing too badly, though.
…but you’ve stopped doing it well? 
I’m about to switch from my second major career incarnation to my third, and all three of them have been “omigod this is what I want to do for the rest of my life.” Since I seem to last about 12-15 years in each, and since I’m now 51, I’m guessing – hoping! – this new one will take me to retirment.
I’m the same way. I can’t count how many times I’ve thought… THIS! This is it, this is what I want to do forever! And then something leadsd me to a new line of work, and then I think “No, wait! This is what I want to do forever.”
I’ve decided it means that I can find happiness in whatever I do, as long as I look at it the right way.
That, or I’m a flake.
I think I was 15 or so when I started drawing seriously. Before I’d had many hobbies I’d got quite good at before I hit the point where I’d seen pretty much all there was to see and, if I wanted to get any better, I’d have to put in way more time and energy while learning very little new material. Every time I’d decided my efforts weren’t worth the rapidly diminishing rewards and lost interest. I was surprised to find after a year of drawing that I wasn’t sick of it yet. I was still learning new things all the time and there was still so much left to master. Soon many online artists I respected got their big breaks and I was there to see the whole process, how they’d improved from “fairly good amateur” to “employable” and how hard they’d worked to get there. I thought, starving artist cliche be damned, as long as you’re at the top of your field there’ll always be work available. I hope I was right, because there’s really nothing I’d rather do.
Just to clarify - I’m not talking about the actual making a living part of it. In my experience that DOES take some, well, experience. And failure.
I’m thinking more along the lines of - I want to teach (no idea who, where or what), versus I love science and want to do research (again, without a clear idea what).
As an example, my sister was clearly a musician at 14. Went to college, almost finished a degree in composition – got the willies. Decided she’s better study something she can turn into a career. Started over again in psychology and now has a freakin PhD. So what’s she doing for a living? She’s a musician. With a lot of loans to pay back.
I wrote a book at the age of 24 and bought a computer to type it on. The book sucked but I found an aptitude and enjoyment for software development. And here I am.
36 here, and still have no idea. I’m pretty sure I don’t want to be a programmer, and have been pretty sure of that since I was about 16.
While I figure out what I really want to do, though, programming pays the bills…
I’m 25 and I have no idea.
When I was little, I wanted to be a singer or a figure skater. (I can still sing. I cannot still figure-skate.)
Then I wanted to be a French teacher.
Then I wanted to be a university professor.
Then I wanted to do research in psychology and possibly be a professor too.
Then I decided I never wanted to work in psychology. Got my degree anyway (because I was too ridiculously close to give up at that point!) and then moved to the opposite coast to study make-up art.
Then I got sick and never finished my make-up art diploma (and they won’t let me back in because they are buttheads. A long story.)
I am trying to save up money to show I’m serious about getting back into school…
All this and lately I’ve been having a persistent fantasy about going back to university and taking linguistics. I would also like to take physics and some more astronomy, but that would be more for fun - my math skills are too poor for me to rely on them for actual work. Singing is a persistent fantasy as well, but I cannot write music or lyrics, so I’ve pretty much already failed at that. I could be a “doo-dooo-WAAAH” background singer though!
All these half-dreams and I currently work in retail, which is absolute hell for me. :smack: I don’t know how to get a good job, or even what a good job actually is.
Really? About age 15. Although it was based on far more superficial reasons (i.e., I need a high-paying job because I’m a spoiled brat) than the reasons I have currently (i.e., intellectually challenging, helping people, and … uh, I need a high-paying job because I’m a spoiled brat)
I was in high school when I first started thinking I wanted to be a doctor. However, I wasn’t completely sure if it was right for me or if I was capable of making it through med school.
I had a lot of doubts even when I was in the midst of applying to med schools. I came close to backing out of my interviews and switching over to applying to dental school instead!
It wasn’t until I finally DID start med school that I became truly confident this is where I belong. Whew.
So I guess I had an idea of what I wanted at 16, but I didn’t really know until about 22. It is definitely a great feeling to feel as if I’m where I’m supposed to be. I hope everyone who is still wrestling with this question also finds their niche. 
What Sampiro said.