SB 8.2.30

tato gajendrasya mano-balaujasāṁ
 kālena dīrgheṇa mahān abhūd vyayaḥ
vikṛṣyamāṇasya jale ’vasīdato
 viparyayo ’bhūt sakalaṁ jalaukasaḥ
Synonyms: 
tataḥ — thereafter; gaja-indrasya — of the King of the elephants; manaḥ — of the strength of enthusiasm; bala — the physical strength; ojasām — and the strength of the senses; kālena — because of years of fighting; dīrgheṇa — prolonged; mahān — great; abhūt — became; vyayaḥ — the expenditure; vikṛṣyamāṇasya — who was being pulled (by the crocodile); jale — into the water (a foreign place); avasīdataḥ — reduced (mental, physical and sensory strength); viparyayaḥ — the opposite; abhūt — became; sakalam — all of them; jala-okasaḥ — the crocodile, whose home is the water.
Translation: 
Thereafter, because of being pulled into the water and fighting for many long years, the elephant became diminished in his mental, physical and sensual strength. The crocodile, on the contrary, being an animal of the water, increased in enthusiasm, physical strength and sensual power.
Purport: 

In the fighting between the elephant and the crocodile, the difference was that although the elephant was extremely powerful, he was in a foreign place, in the water. During one thousand years of fighting, he could not get any food, and under the circumstances his bodily strength diminished, and because his bodily strength diminished, his mind also became weak and his senses less powerful. The crocodile, however, being an animal of the water, had no difficulties. He was getting food and was therefore getting mental strength and sensual encouragement. Thus while the elephant became reduced in strength, the crocodile became more and more powerful. Now, from this we may take the lesson that in our fight with māyā we should not be in a position in which our strength, enthusiasm and senses will be unable to fight vigorously. Our Kṛṣṇa consciousness movement has actually declared war against the illusory energy, in which all the living entities are rotting in a false understanding of civilization. The soldiers in this Kṛṣṇa consciousness movement must always possess physical strength, enthusiasm and sensual power. To keep themselves fit, they must therefore place themselves in a normal condition of life. What constitutes a normal condition will not be the same for everyone, and therefore there are divisions of varṇāśramabrāhmaṇa, kṣatriya, vaiśya, śūdra, brahmacarya, gṛhastha, vānaprastha and sannyāsa. Especially in this age, Kali-yuga, it is advised that no one take sannyāsa.

aśvamedhaṁ gavālambhaṁ
 sannyāsaṁ pala-paitṛkam
devareṇa sutotpattiṁ
 kalau pañca vivarjayet

(Brahma-vaivarta Purāṇa)

From this we can understand that in this age the sannyāsa-āśrama is forbidden because people are not strong. Śrī Caitanya Mahāprabhu showed us an example in taking sannyāsa at the age of twenty-four years, but even Sārvabhauma Bhaṭṭācārya advised Śrī Caitanya Mahāprabhu to be extremely careful because He had taken sannyāsa at an early age. For preaching we give young boys sannyāsa, but actually it is being experienced that they are not fit for sannyāsa. There is no harm, however, if one thinks that he is unfit for sannyāsa; if he is very much agitated sexually, he should go to the āśrama where sex is allowed, namely the gṛhastha-āśrama. That one has been found to be very weak in one place does not mean that he should stop fighting the crocodile of māyā. One should take shelter of the lotus feet of Kṛṣṇa, as we shall see Gajendra do, and at the same time one can be a gṛhastha if he is satisfied with sexual indulgence. There is no need to give up the fight. Śrī Caitanya Mahāprabhu therefore recommended, sthāne sthitāḥ śruti-gatāṁ tanu-vān-manobhiḥ. One may stay in whichever āśrama is suitable for him; it is not essential that one take sannyāsa. If one is sexually agitated, he can enter the gṛhastha-āśrama. But one must continue fighting. For one who is not in a transcendental position, to take sannyāsa artificially is not a very great credit. If sannyāsa is not suitable, one may enter the gṛhastha-āśrama and fight māyā with great strength. But one should not give up the fighting and go away.