Therefore the Vedic system of civilization is seventy-five percent life of celibacy. In the brahmacari system there is no connection with woman. Student life. Student life, if one remains brahmacari, he becomes determined. His brain becomes very receptive. Therefore, in the brahmacari system, complete celibacy, no connection with woman. So up to twenty-five years, if he does not discharge semen, he becomes very stout, strong, and his health is built up for whole life, and he becomes so intelligent that anything he will hear, he will remember immediately. Then after brahmacari system, if one cannot remain brahmacari, naisthika-brahmacari, then the spiritual master allows him to marry. That is grhastha-asrama. So when one is complete, fit for sex life, he begets children, male children, and after twenty-five years, the child becomes grown up, so he retires. In this way, brahmacari, grhastha, vanaprastha, and sannyasa.
From Srila Prabhupada’s lecture on SB 2.3.19, Los Angeles, June 15, 1972