It's not fair, many women whose biological clocks are ticking thinking to themselves, that men just keep on being fertile often into old age! From the time boys start puberty, they just keep on making those swimmers. That's not entirely true. A man's sperm count tends to decrease a bit with age. And, just like the female egg, sperm also have a limited lifespan. Just how long can sperm survive, inside the body? And how does this affect the conception process?
Women are born with all the eggs they will ever have (unless stem cells could produce an "endless supply of eggs" yes, the future is here!). Those eggs mature inside the ovaries and one mature egg is released every month, during ovulation. But sperm, too, go through a whole process before they are ready for use. Testicles produce sperm, after which they mature there. That process can take longer than two months! But, once the sperm leave the male body, along with the rest of the man's seminal fluid, they still have a long journey to make if they want to get to that egg.
If you are trying to conceive and have intercourse around the time of your ovulation (which you can find out with the help of ovulation tests), your sperm has a head start in the form of fertile cervical mucus inside the vagina. Then, it needs to make its way through the cervix, the "door to the uterus", and all the way through the uterus itself into the fallopian tubes. It is actually possible for sperm to survive inside a woman's body for five days, but most stick around for three to four days once they reached the fallopian tubes. It will take a healthy, fast moving sperm to penetrate an egg and fertilize it. Millions of sperm will lose that battle, and if you are lucky only one sperm will survive and contribute to the making of new life.