If you're thinking about getting pregnant, anytime is a good time to start thinking about seeking advice from an infertility specialist and consider counting your eggs. Although an abnormal ovarian reserve test is unexpected in someone under the age of 35, an argument can actually be made to screen the ovarian reserve of all women who are interested in getting pregnant.
In general, since age plays a pretty big role in a woman's ability to conceive, and fertility and pregnancy rates start to diminish around age 35, I usually advise patients who are over 35 or who have been trying to conceive for 6 months without success that they should look into this further by first meeting with an infertility specialist. But in addition to women over 35 or women having trouble trying to conceive, it's also important for women with a history of ovarian surgery, chemotherapy, radiation therapy, severe endometriosis, smoking, or a strong family history of early menopause to be evaluated earlier for diminished ovarian reserve. All of these factors can affect egg quantity and quality; so if you fall into any of the above categories, you might want to consider meeting with a specialist.