Srimad Bhagavatam Canto 11, Chapter 18, Text 11

SB 11.18.11

yadasau niyame ’kalpo
 jaraya jata-vepathuh
atmany agnin samaropya
 mac-citto ’gnim samaviset
If the vanaprastha 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 vanaprastha 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 sannyasa. If one simply cannot carry on his religious duties due to old age, it is here advised that he fix the mind in Lord Krsna 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.
Srimad Bhagavatam Canto 11, Chapter 18, Text 10
Srimad Bhagavatam Canto 11, Chapter 18, Text 12