Chapter 2,Verse 22

vaasamsi jirnani yatha vihaya
navaani grhnaati naroparaani
tatha sharirani vihaya jirnani
anyaani samyaati navaani dehi


This is another important verse in the Gita,where Krishna transcends from his usual reference in the previous verses of referring to the atma as the all pervading,eternal spirit.He now speaks about the 'Invisible body'(Sukshma shariram + Karana shariram),i.e the mind,prana & the karma bearing portion of the human body.He calls this the 'dehi'.He says that just as we discard worn out clothes and wear new ones,the atma decides to abandon the worn out body it has been inhabiting in this janma,and moves into a new body.Once the body has served its purpose,it is considered worn out,be it at the age of 20 or 80.When this takes place is not in our hands.That is decided by our karma.Death is nothing but the transition of the atma from one body to another.