HI, According to Einsteins relativity time is not absoulute physical quantity. Time also depends on velocity of frame of reference. When one of the twin travel in high speed space craft then the time in the spacecraft runs slow in comaprison to the people one earth ( rest frame) but for the twin in spacecraft the time is normal.So in this case there are two diffrent times.one spacecraft time and another earth time which is in rest(lets suppose) with respect to space craft.Now the twin who is in space craft is going to have his entropy change( his physcial change or mental as older or younger) according to the apacecraft time. So if he spent 5 years in space craft accoroding to space craft time then he will be older by 5 years but here in earth more than 5 years (depends on spacefraft speed) will have been elapsed for example 20 years. So if the twin returns to earth after 5 years of spacecraft time then he will find himself 15 years yonger than his twin who remained on earth. He has no idea that time on earth already elapsped by 15 years. IF the time diffrence is quite much then it can be felt physically( he will be indeed younger) or comapring earth time with the spapce craft time.