It's probably your age. Like eiri said, men don't ejaculate from birth. That starts with puberty, which is right about the age that you are at.
For more information on puberty:
"Q. When do most guys have their first ejaculate/have an orgasm? I turn 13 in early November and I haven't had either of those experiences. I know it's normal and all, but I'm just wondering.
A. Most guys probably have their first "wet dreams" around 12-13, or whenever they begin puberty. A "wet dream" is ejaculating while you're asleep, and it's the body's way of releasing extra semen. Unfortunately, we don't feel the orgasm when that happens because we're asleep! Now, around the same time, many boys discover masturbation and it becomes a normal, regular ritual with some (daily, weekly, etc). Now, I won't tell you to do it, but that is when most guys ejaculate for the first time while they're awake."
http://www.puberty101.com/p_masturbationgu
ys.shtml
"Other changes include a deepening of the voice, an increase in muscle mass, the ability to get erections and ejaculate (especially spontaneous nocturnal emissions or 'wet dreams'),..."
http://www.keepkidshealthy.com/adolescent/
puberty.html
You'll probably start to ejaculate any time now. You're about that age.