A lot of people just don't have that, especially in their younger years. You should think yourself fortunate that you have such a clearly defined direction in which you want to travel. Even those with somewhat clear directions can feel lost.

I myself just want to sit around making music all day. However, experimental Drum & Bass isn't going to bring the money in and I'm stubborn when it comes to this. I don't want a part time job, I don't want to have to do things because "that's just how the world is", I want to make my music. You can call me silly, unwise, or immature in that regard and perhaps you'd be correct, but you can see how people have things they'd love to do only to have society make it seem impossible. I also wouldn't mind doing music journalism, SFX/Foley/Music for moving image, or being writer. But again, all very closed off areas which demand you to work for free for a good while.

Fortunately for me, my parent's, especially my farther, work incredibly hard and support my endeavours amazing. Even after paying all my rent and living expenses through university and then paying my rent for 6 further months afterwards, they're still going to lend me the money to do a masters in Sonic Arts. With this I hope to move in to the realm of Lecturing. Not because that's a direction I dream of being in, but because that's one I can see myself enjoying whilst support my true passions. I am of course very grateful for their support.

But some aren't so lucky. If you take a minute to consider all the circumstances beyond your own experience, you'll soon see it's very easy to feel lost. Even if to you it seems silly with your sights as clear as day. Those personality tests also do little. They tell you all the stuff about yourself you already knew and nothing beyond that. But I agree knowing yourself well is certainly a benefit, but that's only half of it.

Edit: To answers the post's question, I'm 22. For some reason I haven't quite figured out yet I still wish to call myself 21.