Of course it's a little late for me, but maybe the method by which I discovered my true calling could be of some use to someone not so...old.
1. Quite by accident, having decided on science as the general theme of my favorite website, I was halfway to my answer without even knowing it.
2. Fast forwarding to today and taking a critical look at my subsequent contributions to my favorite website over the past year--see the LPB and /r/FoS/(?)--and the mystery of what I wanted to do when I grow up is definitively solved.
So what's the answer to the question? Astrobiologist.
But back to the method.
Step One: Look at your list of favorite websites and glean from them a general field of interest. Examples might be entertainment, business, games, food, etc. For me it was science. The key here is you're not forcing it. You've generated a list of favorite websites already. All you're doing is reading into that list what, at the end of the day, you're doing with your free time.Broken down I've simply scrutinized my online habits for clues to what interests me. It took a bit of structure and discipline (editing FoS) for me to be able to narrow it down to the point that I could put my finger on a very specific vocation, but that's just me. Chances are if you scrutinize your own unique virtual meanderings with this question in mind, you'll find that you've already left yourself a trail of bookmarks and favorites leading to what you should be doing with your life.
Step Two: Start keeping track of what content within this general category is most interesting to you. In this sense, I think the means are important. In my case, I wanted to populate a blog and forum with only the most interesting things I came across. So I wasn't just highlighting a list of bookmarks. I was selecting from that list of bookmarks only those items I thought worth sharing (would be interesting to others). Of course I'm still relying on my own discriminating tastes in making these determinations, so the end result was what's most interesting to me. To put it into perspective, what you see on the LPB and /r/FoS/ is only a very small percentage of the science content I've perused over the past year.