Men typically never stop producing sperm, but there is a decline in sperm quality with age. The World Health Organization has set semen parameters which are benchmarks for healthy sperm. These include count, morphology (shape), and motility (movement). Starting around the age of 35, men may see their semen parameters getting worse.
Sperm health can depend on various factors which influence chances of fertility. In terms of quantity, fertility is most likely if the semen discharged in a single ejaculation contains at least 15 million sperm per milliliter.
Too little sperm in an ejaculation might make it more difficult to get pregnant.
When it comes to sperm motility, pregnancy is possible with less than 40% of the sperm in ejaculate moving, with 40% being the threshold since the more the better chances of fertility.