Manava Dharmashastra 6.33

6.33

vaneṣu ca vihṛtyaivaṃ tṛtīyaṃ bhāgam āyuṣaḥ
caturtham āyuṣo bhāgaṃ tyakvā saṅgān parivrajet //

But having thus passed the third part of (a man's natural term of) life in the forest, he may live as an ascetic during the fourth part of his existence, after abandoning all attachment to worldly objects.