An astrological age is determined by what constellation the Sun appears to be in during the Vernal Equinox - the point at which the Sun crosses the celestial equator. Currently this constellation is Pisces.
However, due to the 25,800-year cycle of precession of the Earth's axis of rotation, the time of the Vernal Equinox shifts very slightly every year. This means that - from our perspective - the Sun will shift relative to the background constellations every year, and eventually the Vernal Equinox will occur when the Sun appears to be in the next constellation along the ecliptic, Aquarius.
Astrologers fiercely debate amongst themselves as to when this will be. Some say it occurred in 1447, others say it will occur in 3597, and others cite various dates in between, since astrologers never bothered to define actual borders between the constellations.
Astronomers did this back in 1929 when the International Astronomical Union defined boundaries between areas of space corresponding to each constellation, to be used as a navigational aid for amateur astronomers. Using the IAU definition of the border between Pisces and Aquarius, the Vernal Equinox will cross this line around the year 2600. The next astrological age, Capricorn, will be about 2150 years after that, or 4750.