When a female is born, she is born with all the oocytes she will ever need or have in her entire lifetime. The egg she ovulates every some 27 days will be as old as she is. The earlier she gets pregnant, assuming there are no hereditary defects, the greater the possibility of giving birth to a health child. This is because the egg is exposed to less DNA-altering factors, such as radiation, pollution, toxins, etc. Also, as a woman gets older, there are more complications, such as the likelihood of twins and increased chance of Down's syndrome.