The only answer I have to your question is that, as a man ages, his sperm begins to decrease, when I was young I had plenty of sperm to enter the woman, but as I ages my sperm is much less now! It is like a fuel pump on your car, when it is new and working good, it pumps gas well, but when it gets of an age, the pump begins to get weak and not much fuel will come out.

it's a possibility that it is very little, and those guys swim up there to try to find your egg. but doesnt come back down b/c it wasnt that much in the begining.
this shouldnt be a problem unless your trying to get pregnant.

ServieU,
Now that is a very good answer.
I'm sure this will help her to understand what happens to her husband's sperm once it enters her vagina!

has men get older they can have "retrograde ejaculation". this is where the ejaculate goes into the bladder rather than exiting the penis. this is normal and not harmful, due to a weakening muscle (valve) that stops urine flow while erect.