Matter changes in six phases: birth, growth, maintenance, production of by-products, dwindling, and death. But the life within matter, the spirit soul, is eternal; it goes through no such changes. Life appears to be developing and decaying, but actually it is simply passing through each of these six phases until the material body can no longer be maintained. Then the old body dies, and the soul enters a new body. When our clothing is old and worn, we change it. Similarly, one day our bodies become old and useless, and we pass on to a new body.
As the embodied soul continually passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. This means that only the material body of the indestructible and eternal entity is subject to destruction. This material body is perishable, but the life within the body is nitya, eternal.
Ref: Chapter 6g, Science of Self Realization authored by Srila Prabhupada (Founder – Acharya, ISKCON)