SB 11.18.1

śrī-bhagavān uvāca
vanaṁ vivikṣuḥ putreṣu
 bhāryāṁ nyasya sahaiva vā
vana eva vasec chāntas
 tṛtīyaṁ bhāgam āyuṣaḥ
śrī-bhagavān uvāca — the Supreme Personality of Godhead said; vanam — the forest; vivikṣuḥ — desiring to enter; putreṣu — among the sons; bhāryām — wife; nyasya — entrusting; saha — together with; eva — indeed; — or; vane — in the forest; eva — certainly; vaset — one should dwell; śāntaḥ — with a peaceful mind; tṛtīyam — the third; bhāgam — division; āyuṣaḥ — of life.
The Supreme Personality of Godhead said: One who desires to adopt the third order of life, vānaprastha, 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 vānaprastha order at the age of fifty, and then at the age of seventy-five he may take sannyāsa for complete renunciation. Since in Kali-yuga very few people live for one hundred years, one should adjust the schedule accordingly. Vānaprastha is intended as a gradual transition from materialistic family life to the stage of complete renunciation.