Testes contain a mix of immature, mature, and dying sperm, as well as sperm of varying health and motility.

“Rather, at any given point in time-- like right now, if you have testes, you have some sperm in your testes that are immature and cannot swim, cannot deliver those contents to-- those genetic contents, rather-- to the female egg. And you have some sperm that are mature, and you very likely have some sperm that are so mature that they are dying off or that they're dead. Almost certainly, also, regardless of your age, you have some sperm that are healthier than others, that are better swimmers than others.”