Age is a very important factor for determining a woman's ability to conceive. As women get older, the chances of conception go down dramatically, particularly, as women get past their mid-30s. It's very important that woman see their fertility doctor at this point in time if they haven't conceived within six months after the age of 35.
Younger women, particularly women in their late teens and early 20's have a very excellent supply of eggs, typically. Also, the egg quality tends to be very good. That is, genetically, those eggs are very capable of fertilization and implantation. As women get older, the number of eggs that they have decreases and the genetic quality of those eggs also diminishes making fertility that much more challenging.
In addition to the age affects on the ovaries, there are also age effects on a woman's uterus. Certain uterine conditions like uterine fibroid tumors, which are benign tumors that can affect the uterine cavity and even block the fallopian tubes, increases when women age. It's important that women consider that possibility and seek fertility care when they're younger. As women get into their late-30s the chances of conception drop dramatically and the chances of miscarriage go up dramatically. Some studies have indicated that the vast majority of pregnancies that occur in women over 40 will end in early miscarriage, even if not clinically known to the patient.
In summation, age is a critical factor for achieving pregnancy. Woman who wait to achieve pregnancy until later in their reproductive life need to understand that the chances of conception will diminish and the chances of miscarriage will rise.