SB 11.28.41

kecid deham imaṁ dhīrāḥ
 su-kalpaṁ vayasi sthiram
vidhāya vividhopāyair
 atha yuñjanti siddhaye
kecit — some; deham — the material body; imam — this; dhīrāḥ — self-controlled; su-kalpam — fit; vayasi — in youth; sthiram — fixed; vidhāya — making; vividha — by various; upāyaiḥ — means; atha — thus; yuñjanti — they engage; siddhaye — for the achievement of material perfections.
By various methods, some yogīs free the body from disease and old age and keep it perpetually youthful. Thus they engage in yoga for the purpose of achieving material mystic perfections.

The process described here is meant to fulfill one’s material desires, not to bring one transcendental knowledge. Therefore this process cannot be accepted as devotional service to the Lord, according to Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura. Despite all such mystic perfections, the material body will ultimately die. Actual eternal youth and happiness are available only on the spiritual platform of Kṛṣṇa consciousness.