In the beginning we, especially the brahmanas and the ksatriyas, two classes, higher classes, they have to take sannyasa. If not, at least the brahmanas must take sannyasa. The brahmanas have got four asramas: brahmacari, grhastha, vanaprastha, and sannyasa. The ksatriya have got three asramas: brahmacari, grhastha, vanaprastha, no sannyasa. And the vaisya two asramas: brahmacari, grhastha. Neither vanaprastha or sannyasa. And the sudras, they are simply one, grhastha, that's all. This is the Vedic order.
Lecture on Srimad-Bhagavatam 2.2.5, New York, March 5, 1975