SB 11.17.38

gṛhaṁ vanaṁ vopaviśet
 pravrajed vā dvijottamaḥ
āśramād āśramaṁ gacchen
 nānyathāmat-paraś caret
Synonyms: 
gṛham — the family home; vanam — the forest; — either; upaviśet — one should enter; pravrajet — one should renounce; — or; dvija-uttamaḥ — a brāhmaṇa; āśramāt — from one authorized status of life; āśramam — to another authorized status; gacchet — one should go; na — not; anyathā — otherwise; amat-paraḥ — one who is not surrendered to Me; caret — should act.
Translation: 
A brahmacārī desiring to fulfill his material desires should live at home with his family, and a householder who is eager to purify his consciousness should enter the forest, whereas a purified brāhmaṇa should accept the renounced order of life. One who is not surrendered to Me should move progressively from one āśrama to another, never acting otherwise.
Purport: 

Those who are not surrendered devotees of the Lord must rigidly observe the regulations governing one’s authorized social status. There are four social divisions of life, namely brahmacarya, gṛhastha, vānaprastha and sannyāsa. One who wants to fulfill material desires should become an ordinary householder (gṛhastha), establish a comfortable residence and maintain his family. One desiring to accelerate the process of purification may give up his home and business and live in a sacred place with his wife, as indicated here by the word vanam, or “forest.” There are many sacred forests in India meant for this purpose, such as Vṛndāvana and Māyāpur. The word dvijottama indicates the brāhmaṇas. Brāhmaṇas, kṣatriyas and vaiśyas are all dvija, or initiated in the Gāyatrī mantra, but the brāhmaṇa is dvijottama, or the highest among those who have received second birth by spiritual initiation. It is recommended that a purified brāhmaṇa take to the renounced order of life (sannyāsa), giving up further contact with his so-called wife. The brāhmaṇa is specifically mentioned here, since kṣatriyas and vaiśyas are not to take the renounced order of life. Even so, there are many stories in the Bhāgavatam wherein great kings retire with their aristocratic wives to the forest to practice the austerities of vānaprastha and thus accelerate the process of purification. The brāhmaṇas, however, may directly accept the renounced order of life.

The words āśramād āśramaṁ gacchet indicate that one may progressively move from brahmacārī life to gṛhastha life to vānaprastha life and then to sannyāsa. The words āśramād āśramam emphasize that one should never remain without an authorized social status, nor should one go backward, falling down from a higher position. Those who are not surrendered devotees of the Lord must rigidly observe such injunctions, for otherwise they will quickly become degraded, and their sins will place them outside the bounds of authorized human civilization.

Lord Kṛṣṇa emphasizes here that a nondevotee must rigidly observe the rituals and regulations of Vedic social divisions, whereas the Lord’s pure devotee, engaged twenty-four hours a day in Lord Kṛṣṇa’s mission, is transcendental to such divisions. If, however, one performs illicit activities on the strength of being transcendental to Vedic social divisions, one is revealed to be a materialistic neophyte and not an advanced devotee of the Lord. An advanced devotee, who remains aloof from material sense gratification, is not bound by the Vedic social divisions; thus even a householder may live very austerely, traveling and preaching Kṛṣṇa consciousness away from home, and even a sannyāsī may sometimes engage women in the devotional service of Lord Kṛṣṇa. The most advanced devotees cannot be restricted by the rituals and regulations of the varṇāśrama system, and they move freely around the world distributing love of Godhead. Mat-para indicates a pure devotee of the Lord who always keeps the Lord fixed in his heart and consciousness. One who falls down to become a victim of sense gratification is not fully established on the platform of mat-para and should rigidly observe the social divisions and regulations to remain steady on the platform of pious human life.