The human form is destined to live for an average lifespan of 100 years. That is what Hinduism scriptures say. To live beyond 100 years is an added bonus for some, others suffering due to various diseases want to quit early but cannot. Euthanasia not permitted in many countries.
The flailing human life that sometimes extinguishes in childhood when only few months old, how could the human body survive for eternity! Most parts of human body start failing beyond 70 years of age. All depends on our daily routines and indulgence in physical activity.
Immortality always belonged to our true inner self existing in heart as our soul atman, the spirit. It is this spirit that manifest human form to work out its karma, remove dross impurities within. From the beginning of life souls atmans were immortal indestructible, could not be created or destroyed.
Imagine a human being in old age with a flailing body and yet aspiring to become immortal.
Immortality always belonged to spiritual travelers like Mahavira, Gautama Buddha, Jesus Christ and Prophet Mohammed. These man gods in their lifetime indulged in spirituality to reach state of enlightenment followed by moksha salvation. Physically all are dead but immortal through their teachings. Enlightened souls never die but live amongst us in their teachings.
We can become immortal by indulging in spirituality, gaining enlightenment at the end of journey.