An astrological age is defined by the constellation of the zodiac in which the vernal equinox lies. Astrological ages change because of a phenomenon known as the precession of the equinoxes. One complete period of this precession is about 25,925 years. An Astrological Age is defined as the period of time during which the Vernal Equinox Point can be found in a particular constellation.
The concept of the Astrological Age was most carefully defined by the celebrated psychologist Carl Gustav Jung who in the 1950s popularised the Age of Aquarius.
In astrology the astrological age is defined by the constellation in which the Sun appears during the vernal equinox. Since each sign of the zodiac covers 30 degrees, each astrological age might be thought to last 70 × 30 = 2,100 years. This means the Sun crosses the equator at the vernal equinox moving backwards from year to year at the rate of one degree in seventy-two years, one constellation in about 2156 years, and the whole twelve signs in about 25 868 years (called a Great Sidereal Year).
The current astrological Age of Pisces began about 500 A.D., since it was the last time that, astronomically, the vernal equinox occurred in the first point of the constellation Aries leaving it and entering the constellation Pisces. Nowadays, the vernal equinox occurs astronomically in about nine degrees of the constellation Pisces and it will be only about 2600 A.D. it actually finishes moving backwards through all the 30 degrees of Pisces and enters the constellation Aquarius.
An astrological age varies, because it is the amount of time taken for the the Vernal Equinox Point to move all the way through a particular constellation of fixed stars and the constellations come in a variety of sizes. Hence, the Astrological Age associated with a large constellation, Virgo, the Maiden, for example, is much longer than that for a small constellation, such as Scorpius, the Scorpion.
Altogether there are thirteen Astrological ages. Because the Vernal Equinox Point will pass through thirteen Constellations during the course of a Great Year.
An Astrological Age for a particular constellation begins when the Vernal Equinox Point moves into that constellation, as seen from Earth. It ends when the Vernal Equinox Point moves out of that constellation again, as seen from Earth. So it will be the Age of Aquarius when the Vernal Equinox Point moves out of the constellation of Pisces and into the constellation of Aquarius.