Hemospermia, blood in the ejaculate, is a not uncommon occurrence in sexually active males. The work-up consists of a urological physical exam and seldom is the cause found. The bleeding usually stops on it's own. What you describe is blood from your urethra BEFORE ejaculating, and may represent a different problem. It seems to me that this would be coming from the glands along the urethra that secrete before the orgasm ('pre-***"). Your work-up seems reasonable as you describe it. I would make sure that the CAT scan included the pelvis down through the level of your rectum and I would suggest a culture of your semen as well as an evaluation of your blood clotting mechanism (blood test).
If nothing is found, there is nothing to do except see if it stops and get re-examined in 6 months. I do not see how age figures into it.
S.A.Liroff, M.D.