So, I'm 14, going on 15. And by vibrato, I mean of the voice (vocal cords?). Anyway, a good description of it would be that it just came out of the blue. I was singing to myself one day, maybe a year ago, and it just happened. I noticed soon after developing vocal vibrato that I developed "humming" vibrato. That makes sense, I suppose.
My problem is, unless I really force it - and it doesn't feel "right" when I do - I can't not vibrate. In other words, I vibrate whether I want to or not. I recognize that it is (relatively) rare for someone to be born with unnoticed and undeveloped natural vibrato, let alone for someone who really doesn't sing that much at all. I'm in orchestra, not choir.
What I'd really like to do is to be able to control when I vibrate. I can stop just fine while I'm humming, but, again, if I'm singing I can't. Also, whether humming or singing, I cannot control the frequency of the vibrato - I'd really like to slow it down, because I clocked myself at 10-12 Hertz, and once at 16 Hz.
Just a note: this is not "artificial" vibrato. I've examined the sensations closely, and I can say for a fact that I am most definitely not moving my stomach or diaphragm to accomplish vibration, and I'm not moving my jaw either. It's a pure, natural, yet unconscious and uncontrollable vibration.