I'm fourteen. I masturbate kind of frequently, but I haven't seen any semen come out. Why is this?
It is simply because you haven't start producing semen yet. But that also means you are masturbating, not because of need but merely because it feels good. That is a dangerous position. It is like the person who eats because he enjoys food and not because he needs fuel for his body; eventually he falls into gluttony. When a guy chases after sexual pleasure merely for the pleasure he will soon fall to all sorts of sexual sins. "And He said, "What comes out of a man, that defiles a man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man"" (Mark 7:20-23).
Instead of trying to speed things up and make things happen, just things take its natural course. There are plenty of more important things to do.