In ancient times, so the Vedas report, the demons were once equals of the gods in every way…

But their disdain for serving anyone other than their own selves grew so strong that it polluted their performance of Vedic dharma. Verse 26 of Mahabharata 7.221 tells us that the demons used to be firm adherents of dharma: asurenvavasam purva satyadharmanibandhana. They followed svargamarga, the pathway to heaven (verse 28), they gave charity, they performed sacrifices, they worshiped guru and gods, and they showed hospitality to learned brähmanas (verse 29). But in time lust and anger covered these virtues. Mahäbhärata 3.92.6 says that during a period of history known as the Deva-yuga, the asuras (demons) became distinct from the demigods at the moment they abandoned dharma. Laksmi (the goddess of good fortune) left them, and Alaksmi (the goddess of misfortune) became their constant companion (verse 9). Verse 10 states:      

tän alaksmi samävistän darpopahatacetasau
daiteyän dänaväs caiva kalir apy äviçat tatau

Kali entered the demons, whose minds were afflicted-
with pride and who were surrounded by Alaksmi.

Kali (a male personage, never to be confused with goddess Käli) is quarrel personified. His family lineage is described in Srimad-Bhagavatam 4.8.2-3—it begins with Brahmä, the creator, and soon comes under the shelter of Niriti, the goddess of the southwest who is associated with untimely death, difficulty, poverty and infertility. Kali consorted with his own sister, Durukti (Harsh Speech), and begot in her two children, Bhaya (Fear) and Mrityu (Death). Beside quarrel, Kali brings with him irreligion, greed, falsehood, robbery, incivility, treachery, misfortune, cheating, and vanity (Srimad-Bhagavatam 1.17.32). By his superior skill as an agent of suffering, Kali became the leader of the asuras as much as Brahmä is the leader of the demigods by his superior Vedic knowledge (asurau kali-prayanta evaa duukhottarottaräu kalir duukhädhikas tenu te'py eva brahmavad ganau).
Kali is ever on the lookout for discrepancies in a person's execution of dharma. When, for example, the pious King Nala forgot to wash his feet after going to the toilet, 17 and then sipped water and performed his sandhyä rituals, Kali entered his body and pulled him down to ruination. After a terrible struggle Nala at last got free of Kali's clutches and recovered his former status, but those who deliberately abandon dharma—the demons, in other words—willingly follow Kali into the moral abyss.
The archetypical mleccha—an uncivilized person of abominable culture—this Kali has his own age, Kali-yuga, a time when mlecchas overtake the earth. That age began five thousand years ago. During his yuga Kali has permission from the Supreme Lord to promote immorality everywhere. The history of how Kali came to get this license is as follows.
.         In the begining earthly mleccha population was practically non existing. Kali and his wife, desiring the welfare of his race, worshiped Lord Visnu. The Lord appeared and assured Kali that he would have his own yuga (age). During this period lasting 432,000 years, all of Kali's desires would be satisfied. When Kali asked how the population of mlecchas would increase in preparation for his age, the Lord indicated that a man named Adama and his wife Havyavati would spawn a new class of untouchables when the time was right.
Adama lived with his wife in a great forest. Both were pious souls. Nearby grew a päpa-vriksa or a sinful tree. Kali came there and assumed the form a serpent. He enticed Adama to eat the päpa-vriksa fruit. After that, Havyavati became pregnant and gave birth to mlecchas. The sinful population spawned by Adama and Havyavati gradually increased in number. By nature, mlecchas are addicted to illicit sex—in other words, sex that is indulged in for the purpose of gratifying the senses rather than the procreation of good children. In Vedic culture, sexuality was governed by the garbhädäna-saaskära, by which a husband and wife begot children in a sattvic, good atmosphere. Children born of illicit sex are infected by rajo-guna (passion) and tamo-guna (ignorance). They are called varna-sankara, or undesirable progeny. Their natural inclination is to overturn social traditions by acts of immorality, criminality and violence.
By the time of Lord Krishna's appearance on earth some five thousand years ago, the mleccha population was large enough for an untouchable king named Kälayavana to muster a large army that attacked the Lord's city at Dvärakä. Kälayavana was burned to ashes by the glance of the devotee Mucukunda.
Visnu Purana 4.24.115 states:

yasmin krishno divya yätas tasmin
eva tadähani pratipanna kali-yugam.

                 The day and the moment when Lord Krishna left for His divine abode,
the Age of Kali was established on earth.

The exact date is 20 February 3102 BC.

This is an excerption from Suhotra Swami’s book “Dimensions of good end evil”