ekada vainya atmavan
jagatas tasthusas capi
vrttido dharma-bhrt satam
yad-artham iha jajñivan
atmajesv atmajam nyasya
virahad rudatim iva
prajasu vimanahsv ekah
sa-daro ’gat tapo-vanam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
At the last stage of his life, when Maharaja Prthu saw himself getting old, that great soul, who was king of the world, divided whatever opulence he had accumulated amongst all kinds of living entities, moving and nonmoving. He arranged pensions for everyone according to religious principles, and after executing the orders of the Supreme Personality of Godhead in complete coordination with Him, he dedicated his sons unto the earth, which was considered to be his daughter. Then Maharaja Prthu left the presence of his citizens, who were almost lamenting and crying from feeling separation from the King, and went to the forest alone with his wife to perform austerities.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maharaja Prthu was one of the saktyavesa incarnations of the Supreme Personality of Godhead, and as such he appeared on the surface of the earth to execute the orders of the Supreme. As stated in Bhagavad-gita, the Supreme Lord is the proprietor of all planets, and He is always anxious to see that in each and every planet the living entities are happily living and executing their duties. As soon as there is some discrepancy in the execution of duties, the Lord appears on earth, as confirmed in Bhagavad-gita (4.7): yada yada hi dharmasya glanir bhavati bharata.
Since there were so many discrepancies during the reign of King Vena, the Lord sent His most confidential devotee Maharaja Prthu to settle things. Therefore, after executing the orders of the Supreme Personality of Godhead and settling the affairs of the world, Maharaja Prthu was ready to retire. He had been exemplary in his governmental administration, and now he was to become exemplary in his retirement. He divided all his property amongst his sons and appointed them to rule the world, and then he went to the forest with his wife. It is significant in this connection that it is said that Maharaja Prthu retired alone and at the same time took his wife with him. According to Vedic principles, when retiring from family life one can take his wife with him, for the husband and wife are considered to be one unit. Thus they can both combinedly perform austerities for liberation. This is the path that Maharaja Prthu, who was an exemplary character, followed, and this is also the way of Vedic civilization. One should not simply remain at home until the time of death, but should separate from family life at a timely moment and prepare himself to go back to Godhead. As a saktyavesa incarnation of God who had actually come from Vaikuntha as a representative of Krsna, Maharaja Prthu was certain to go back to Godhead. Nonetheless, in order to set the example in all ways, he also underwent severe austerities in the tapo-vana. It appears that in those days there were many tapo-vanas, or forests especially meant for retirement and the practice of austerities. Indeed, it was compulsory for everyone to go to the tapo-vana to fully accept the shelter of the Supreme Personality of Godhead, for it is very difficult to retire from family life and at the same time remain at home.