SB 4.8.32

ato nivartatām eṣa
 nirbandhas tava niṣphalaḥ
yatiṣyati bhavān kāle
 śreyasāṁ samupasthite
Word for word: 
ataḥ — hereafter; nivartatām — just stop yourself; eṣaḥ — this; nirbandhaḥ — determination; tava — your; niṣphalaḥ — without any result; yatiṣyati — in the future you should try; bhavān — yourself; kāle — in due course of time; śreyasām — opportunities; samupasthite — being present.
Translation: 
For this reason, my dear boy, you should not endeavor for this; it will not be successful. It is better that you go home. When you are grown up, by the mercy of the Lord you will get a chance for these mystic performances. At that time you may execute this function.
Purport: 

Generally, a thoroughly trained person takes to spiritual perfection at the end of his life. According to the Vedic system, therefore, life is divided into four stages. In the beginning, one becomes a brahmacārī, a student who studies Vedic knowledge under the authoritative guidance of a spiritual master. He then becomes a householder and executes household duties according to the Vedic process. Then the householder becomes a vānaprastha, and gradually, when he is mature, he renounces household life and vānaprastha life also and takes to sannyāsa, completely devoting himself to devotional service.

Generally, people think that childhood is meant for enjoying life by engaging oneself in sports and play, youth is meant for enjoying the company of young girls, and when one becomes old, at the time of death, then he may try to execute devotional service or a mystic yoga process. But this conclusion is not for devotees who are actually serious. The great sage Nārada is instructing Dhruva Mahārāja just to test him. Actually, the direct order is that from any point of life one should begin rendering devotional service. But it is the duty of the spiritual master to test the disciple to see how seriously he desires to execute devotional service. Then he may be initiated.