In the \"twin paradox,\" explain (in terms of heartbeats, physical and mental activities, and so on) why the younger returning twin has not lived any longer than her own proper time even though her stay-at-home brother may say that she has. Hence explain the remark: \"You age according to your own proper time.\"

Time dilation applies not only to elementary particles but to all naturally occurring time intervals including pulses rates and human life times. This fact has been used to propose an apparent puzzle that has become known as the twin paradox.
Consider two twin brothers. One is travelling in a high speed rocket and the other remains in the Earth from an instant. During the journey, the other twin who is travelling in a high speed rocket is able to monitor his heart beat and average respiration rate. He finds himself to be slower than the other twin brother who remains in the Earth by the time dilation effect. Thus, the entire aging process has been slowed down. His brother therefore expects that, upon her return to the Earth after the journey, he will be younger than his twin brother. From the above observation, we conclude that the age of the other twin which travels at high speed platform slowed down to the proper time.