Srimad Bhagavatam Canto 11, Chapter 18, Text 01

SB 11.18.1

sri-bhagavan uvaca
vanam viviksuh putresu
 bharyam nyasya sahaiva va
vana eva vasec chantas
 trtiyam bhagam ayusah
The Supreme Personality of Godhead said: One who desires to adopt the third order of life, vanaprastha, should enter the forest with a peaceful mind, leaving his wife with his mature sons, or else taking her along with him.
In Kali-yuga a human being generally cannot live more than one hundred years, and even this is becoming most unusual. A man who has a reasonable expectation of living for one hundred years may adopt the vanaprastha order at the age of fifty, and then at the age of seventy-five he may take sannyasa for complete renunciation. Since in Kali-yuga very few people live for one hundred years, one should adjust the schedule accordingly. Vanaprastha is intended as a gradual transition from materialistic family life to the stage of complete renunciation.
Srimad Bhagavatam Canto 11, Chapter 18, Text 02