Yugas or ages are explained in the book "The Holy science" by Swami Sri Yukteswar.

Science has explained that the Moon revolves around the Earth and the Earth rotating on an axis, revolves around the Sun. Similarly, the Sun revolves around its dual and also revolves around the grand center called "Vishnunabhi". This is the seat of creative power, Brahma. Brahma regulates dharma or mental virtue of the internal world. When the Sun is nearest the grand center, the mental virtue is at its peak. Men can easily understand all, even mysteries of Spirit. Similarly, when the Sun is farthest from the grand center, it is difficult for men to comprehend more than gross material (physical) creation. This change in mental virtue is gradual. It takes 12000 years for the Sun to move from a point closest to the grand center to the farthest point. This period of 12000 years constitutes a Daiva Yuga and is the descending arc. Likewise, it takes another 12000 years for the mental virtue to develop to the fullest. This is the time taken for the Sun in its orbit to move from the farthest to closest point with respect to the grand center. These 12000 years is the ascending arc of Daiva Yuga. So it takes 24000 years for the Sun to complete one revolution around the grand center.

There are four yugas in one Daiva Yuga. They are Kali Yuga, Dwapara Yuga, Treta Yuga and Satya Yuga. Kali Yuga lasts for 1/20th of the revolution i.e., duration of Kali Yuga is 1200 years. The mental virtue is at its lowest and men are unable to comprehend more than the gross material creation. According to Swami Sri Yukteshwar, the Sun was at the farthest point at 499 AD. The period of 2400 years after Kali Yuga is called Dwapara Yuga. In Dwapara Yuga, the mental virtue grows and men are able to comprehend fine matters or electricities and their attributes which are the creating principles of the external world. The next 3600 years is called Treta Yuga. The mental virtue development is in the 3rd phase. Human intellect is able to comprehend the Divine magnetism, the source of all electrical forces on which the creation depends for its existence. The next 4800 years is called Satya Yuga. Dharma is at its 4th phase and completes development. Human intellect can easily comprehend all, even God, the Spirit beyond this visible world. In the descending Daiva Yuga the human intellect slowly goes down.

A quick glance on the Yuga and its duration:
24000 Years - Time taken for the Sun to complete one revolution around the grand center
12000 Years - 1 Daiva Yuga