A Bazi chart is made up of the Year, Month, Day, and Hour Pillars—each reflecting a unique combination of Heavenly Stems and Earthly Branches at the moment of birth. The Major Luck Cycles (Da Yun) are the timeline astrologers use to observe life’s shifting fortunes, with each step spanning roughly ten years. The "starting age" is simply the point when your first Major Luck Cycle begins. Before that, your life is thought to be more strongly influenced by your Month Pillar and the original chart itself. Once you enter a Major Luck Cycle, you begin stepping through each decade-long phase. So the first step in calculating your Major Luck Cycles is determining your starting age—then everything else follows. Understanding this starting point is key to grasping the entire system.

The key to calculating your Bazi starting age lies in the distance between your birth moment and the nearest Solar Term (Jie Qi). Traditionally, astrologers follow a rule: for a Yang-year male or a Yin-year female, count forward to the next Solar Term; for a Yin-year male or a Yang-year female, count backward to the previous Solar Term. Once you have the total number of days from birth to the relevant Solar Term, you convert it using a ratio of three days equals one year, and one day equals four months. For example, if the distance is nine days, that means you start your first Major Luck Cycle at around 3 years old. Ten days equals about 3 years and 4 months. Although this method may seem intricate, its logic is consistent and has been used in traditional astrology for centuries—showing the ancients' keen observation of time and seasonal shifts.

Whether you count forward or backward depends on your gender and the yin/yang quality of your birth year. In Bazi, the Heavenly Stem of the birth year determines this: years with Heavenly Stems Jia, Bing, Wu, Geng, Ren are Yang years; Yi, Ding, Ji, Xin, Gui are Yin years. The rule: a male born in a Yang year and a female born in a Yin year count forward (to the next Solar Term). A male born in a Yin year and a female born in a Yang year count backward (to the previous Solar Term). This not only affects the number of days used for the starting age but also determines the direction of the Stem-Branch sequence for every future Major Luck Cycle. That’s why two people born in the same month and year but of different genders may have completely different starting ages and cycle directions. Understanding this principle is essential for an accurate reading—otherwise, you could end up with the wrong results.