BHagvad Gita : Chapter 2(Verse 13) 

देहिनोऽस्मिन्यथा देहे कौमारं यौवनं जरा |
तथा देहान्तरप्राप्तिर्धीरस्तत्र न मुह्यति || 13||
"Just as the embodied soul continuously passes, in this body, from childhood to youth to old age, similarly, the soul passes into another body at death; a sober person is not bewildered by such a change."
The meaning of this verse is that just as the soul experiences different stages of life (childhood, youth, and old age) within the same body, similarly, after death, the soul transitions from one body to another. A wise person, who understands the eternal nature of the soul, is not disturbed by this transition.