A woman’s biological clock can be thought of as a hypothetical ticking clock that is counting down to the final timeframe in which she can have children. Any pregnancy after a woman turns 35 is medically defined as a geriatric pregnancy. Every woman is born with all of the eggs she will have in her lifetime — close to 2 million — but she will only ovulate 300 to 400 of them between puberty and menopause. As you age, eggs begin to lose quality and eventually die off. In fact, a woman's fertility begins declining in her late 20s and continues to take dramatic dips for the next 10-15 years, with 40-year-old women having only a 5% chance of conception each month. Different factors other than physical health will ultimately be the determining factor in your ability to conceive. However, despite the fact that fertility declines dramatically at a certain age which is considered to be 35. The quality of eggs does decline over time, fertility treatment options like IVF might not be the best option for women trying to conceive past 35. IVF uses a woman's own eggs, so even if she is undergoing treatment, there is still a chance of birth defects and miscarriages, which are mainly caused by older eggs. A method to circumvent this aging of eggs, is to freeze them. The egg-freezing process begins by starting with a hormonal medication that causes the body to release more eggs than it normally would during a cycle. Then, on the day of the procedure, the doctor uses a fine needle and an internal ultrasound probe to harvest and collect eggs from the ovaries. Finally, the eggs are treated with cryoprotectants to maintain their structure and are frozen into glasslike cells to be stored in a cryopreservation tank. In addition, many lifestyle factors can play an indefinite role in fertility. Women who are overweight are less likely to ovulate each month. Obesity is also linked to miscarriage, gestational diabetes, preeclampsia, and other pregnancy complications, but research shows that a drop in even 5% to 10% of body fat can improve chances of fertility. Additionally, women who smoke and take part in recreational drugs are three times less likely to get pregnant quickly compared to women who don't smoke or do drugs. In conclusion, a biological clock isn’t definite. There are many factors that play into a woman's fertility, and it's up to every individual to decide when and how to be proactive about their reproductive health.
Ref:
https://www.cuimc.columbia.edu/news/your-biological-clock-may-be-older-you-think 
