I think that's because I rediscovered masturbation almost exactly at the point when I started being able to ejaculate. 'Hmm, what's that? I'll try that again tomorrow and test it...' Yep, uses for chemistry sets the makers never thought of.
At that stage, the ejaculate was clear and orgasms felt like a tingle down my spine. It took a few months for it to turn white (presumably my prostrate starting to produce its contribution) and for orgasms to feel even nicer, and more in my head and genitals.
So I ended up doing it several times a day from that point. No wonder I never had any wet dreams!
I have come from fantasy alone a few times though, so I can see how it can happen. When it has, the ejaculation's been almost painful, as though it comes as a surprise. Is that also true for the men who've had them, ie is this what wakes people up?
It is interesting that the people who only started having them after 19 are all women.