I am trying to get through Special & General Relativity on my own. However I seem to reach a problem. Imagine person A standing on earth and person B flying around earth close to speed of light (it doesnt have to be close to speed of light, I just want to make the effects of relativity obvious). Now person A ages by 50 years and person B only by couple of minutes. However whats stopping the Person B from claiming that HE is at rest and person A is moving near the speed of light and thus HE should be the one with gray hair and grand children? I tried to research it and I found that that of course, nothing stops him from saying that, and he would be also right to state it, but I dont seem to get the reason why is that? Clearly one is young and one is old at the end of the flight but how is that possible? Shouldnt they both be young and old at the same time?