New cells for old
The age of eggs and sperm can be very different
Men produce sperm cells continually after puberty, and dispose of old ones after a couple of months. Human eggs (ova), by contrast, are made in the embryo and released from the ovaries much later on. Actually, these are primary oocytes, which have not yet completed meiosis. After puberty, one oocyte a month matures into a secondary oocyte, which can then be fertilised.
The older a woman, the older her eggs, and the greater the chance that she will have a baby with chromosomal abnormalities. Serious chromosomal problems may lead to miscarriage, so older women are at greater risk of this than younger women. Although some studies indicate that there are stem cells in mammalian ovaries that can produce new egg cells in adults, other studies dispute this, and this topic is still highly debated. With more conclusive research, this might be used to help treat infertility or to reduce the risk of some birth defects.