From childhood to old age, even if the body changes, "I" continue.
Within a single lifetime, the body changes from childhood to youth and then to old age. The appearance and sensations change, but through these changes, we feel that we are still the same person.
Krishna explains that the soul, which possesses a body, transmigrates to another body through death, using these familiar changes as clues. Instead of viewing death solely as a sudden disappearance, it is seen as a process in which the soul continues beyond bodily changes, just like from childhood to youth. Those who understand this are taught not to be swayed by bodily changes.
Japanese Translation from the Geeta Press Edition - Chapter 2, Section 13: "Childhood, Youth, Old Age, and Another Body"
The one who demonstrated that the soul is, by its very nature, eternal, reveals its unchanging quality in the following passage. By doing so, it proves that lamenting about the soul is an error.
Sanskrit original text
देहिनोऽस्मिन्यथा देहे कौमारं यौवनं जरा ।
तथा देहान्तरप्राप्तिर्धीरस्तत्र न मुह्यति ॥ १३ ॥Romanization
dehino 'smin yathā dehe kaumāraṃ yauvanaṃ jarā |
tathā dehāntaraprāptir dhīras tatra na muhyati || 13 ||The meaning and translation of section 13 will continue on the next page.
Just as, through this body, childhood, youth, and old age are returned to the soul, so too, another body is bestowed upon the soul.
The wise person does not waver about this matter. (13)
Those ignorant people who consider the soul to be something that undergoes change believe that when the soul leaves a body, or at the time of rebirth, it experiences great suffering, and therefore they lament. In this passage, the Lord declares that this is not wise.
The text states that childhood, youth, and old age actually belong only to the coarse physical body, not to the soul, and that they are mistakenly attributed to the soul. It also says that when a person leaves one body and takes another, it is not actually the soul doing so, but rather a subtle body experiencing those experiences, which is then attributed to the soul.
Only ignorant people lament the transition from one body to another. A wise person never does so. For in the eyes of a wise person, the soul has no connection with the body. Therefore, Arjuna should not grieve for those relatives.