Awesome question about cum!  Guys start to produce thousands of sperm a minute (the average ejaculation has between 300 to 500 million sperm) during puberty.  However, there is some variation based on when you start puberty and how far along you are.  It is VERY common to have erections–you have probably been having them from the time you were a baby–but not ejaculate.  By age 14, you are probably having lots of erections which are out of your control— you are not trying to get one, it just happens.  It is totally normal to not ejaculate at these times.  If you are interested in ejaculating, try gently masturbating and will see if your body has gotten to that point.