Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 11
यदासौ नियमेऽकल्पो जरया जातवेपथु: ।आत्मन्यग्नीन् समारोप्य मच्चित्तोऽग्निं समाविशेत् ॥ ११ ॥
yadāsau niyame ’kalpojarayā jāta-vepathuḥātmany agnīn samāropyamac-citto ’gniṁ samāviśet
If the vānaprastha is overtaken by old age and because of his trembling body is no longer able to execute his prescribed duties, he should place the sacrificial fire within his heart by meditation. Then, fixing his mind on Me, he should enter into the fire and give up his body.
Since the process of vānaprastha is recommended for those nearing the end of life, there is always the likelihood that one will prematurely succumb to the symptoms of old age and be unable to reach the final stage of sannyāsa. If one simply cannot carry on his religious duties due to old age, it is here advised that he fix the mind in Lord Kṛṣṇa and enter into the sacrificial fire. Although this may not be possible in the modern age, we can appreciate the absolute seriousness of going back home, back to Godhead, as evidenced in this verse.