Kumaradhara Thirtham is believed to be the place where Lord Vishnu took Tapas after killing demon Tarakasur (Son of Vajranga and Varangi).
Once there lived an aged Brahmin sage performing tapas at the Venkata Hills. He had an assistant by name Koundinya. One day the sage went out into the deep forest for fetching fruits. On his return he missed his way. He wandered quite some distance in the forest. He called out his assistant's name so that he may hear and come to this rescue. By this he became very thirsty as well as tired.
The Graceful Lord Sri Vishnu assumed the form of a young boy and approached the sage. He spoke to the old man, "O sage! Whom are you calling out in this wilderness. Are there anyone nearby? Should you live at this very ripe age on the hill alone? Is there any meaning for it?
The Sage Replied : I know I have some work for the angels to do. Until I complete them. I should stay alive. But personally I have no ambitions to live any more.
Then the young boy led him into the lake and asked the old man to have a dip in the lake. The old man bathing into the lake was transformed into a young boy of sixteen years. Then the Lord showered his blessings on him and disappeared. From that day onwards the lake/pond was know as "Kumara Thirtham" because it had the power of conferring youth fullness on the aged Brahmin sage.