SB 9.18.39

mātāmaha-kṛtāṁ vatsa
 na tṛpto viṣayeṣv aham
vayasā bhavadīyena
 raṁsye katipayāḥ samāḥ
Word for word: 
mātāmaha-kṛtām — given by your maternal grandfather, Śukrācārya; vatsa — my dear son; na — not; tṛptaḥ — satisfied; viṣayeṣu — in sex life, sense gratification; aham — I (am); vayasā — by age; bhavadīyena — of your good self; raṁsye — I shall enjoy sex life; katipayāḥ — for a few; samāḥ — years.
Translation: 
My dear son, I am not yet satisfied in my sexual desires. But if you are kind to me, you can take the old age given by your maternal grandfather, and I may take your youth so that I may enjoy life for a few years more.
Purport: 

This is the nature of lusty desires. In Bhagavad-gītā (7.20) it is said, kāmais tais tair hṛta-jñānāḥ: when one is too attached to sense gratification, he actually loses his sense. The word hṛta-jñānāḥ refers to one who has lost his sense. Here is an example: the father shamelessly asked his son to exchange youth for old age. Of course, the entire world is under such illusion. Therefore it is said that everyone is pramattaḥ, or exclusively mad. Nūnaṁ pramattaḥ kurute vikarma: when one becomes almost like a madman, he indulges in sex and sense gratification. Sex and sense gratification can be controlled, however, and one achieves perfection when he has no desires for sex. This is possible only when one is fully Kṛṣṇa conscious.

yadavadhi mama cetaḥ kṛṣṇa-pādāravinde
 nava-nava-rasa-dhāmany udyataṁ rantum āsīt
tadavadhi bata nārī-saṅgame smaryamāne
 bhavati mukha-vikāraḥ suṣṭhu-niṣṭhīvanaṁ ca

“Since I have been engaged in the transcendental loving service of Kṛṣṇa, realizing ever-new pleasure in Him, whenever I think of sex pleasure, I spit at the thought, and my lips curl with distaste.” Sexual desire can be stopped only when one is fully Kṛṣṇa conscious, and not otherwise. As long as one has desires for sex, one must change his body and transmigrate from one body to another to enjoy sex in different species or forms. But although the forms may differ, the business of sex is the same. Therefore it is said, punaḥ punaś carvita-carvaṇānām. Those who are very much attached to sex transmigrate from one body to another, with the same business of “chewing the chewed,” tasting sex enjoyment as a dog, sex enjoyment as a hog, sex enjoyment as a demigod, and so on.