Bhagavad Gita 2.13
देहिनोऽस्मिन्यथा देहे कौमारं यौवनं जरा । तथा देहान्तरप्राप्तिर्धीरस्तत्र न मुह्यति ॥
dehino 'smin yathā dehe kaumāraṃ yauvanaṃ jarā | tathā dehāntara-prāptir dhīras tatra na muhyati ||
Translation
As in this body, the embodied one passes through childhood, youth, and old age, so does it pass into another body. The steady-minded one is not deluded about this.
Reflection
What did your younger self lose track of that you, the same one, are still carrying?
Read this verse in its chapter: Chapter Two
The argument by analogy. You have already lived through the death of two bodies: the child's body, the youth's body. The man you are now is the third body you have inhabited. Childhood did not survive into adolescence; adolescence did not survive into adulthood. You did. Krishna uses the most ordinary fact of aging to undo the most absolute fear of death. Dhīra, the steady one, is the word for the person who does not flinch at this. Notice the door: the move from childhood to youth is offered as the same kind of door as the move out of this body. Not different in kind. Different in scale.