Since there is no gravity there are fewer signs of aging. For instance, there will be less wrinkles on your face and less sunspots which are common on older people. Also since we measure our age in days (or ONE rotation around the earth's axle) the exact age of a person will be unknown. So if there were twins and one went to live in space and one stayed on earth and they reunited in 20 years, the one in space would look much younger.

Yes and no. If you were in a spaceship, travelling at almost the speed of light, left earth and returned, relatively speaking, people on earth would have aged a little more than you have.So in effect, less time would have taken place in your journey, for you...than time transpired on earth. Stephen Hawking the astro physist actually explains this better than I could, in his book A Brief History In Time. It has to do with Einstein's theory of relativity. So yes, you are younger relative to how old you would be if you had not gone on that trip. As for the no part of my answer. There are a few things that happen to us in a weightless environment that are bad. If that spaceship didn't have some form of artifical gravity we'd develop weaker bones (osteoporosis .sic). Our heart muscles would weaken as well as others in the body. The bone loss, I believe is permanent, the muscle can be built up again I believe. In any event, you'd have the bones of an old person when you got back, even though you were younger relatively. Lol...what a mouthful.

A real answer: unless a baby is taken into outer space and remains to the age 50, for comparison, no one really knows the answer to your question. Everything else, is just a theory.

All theoretical physics aside, the benefits to mankind in space as well as the detractions all have to do with gravity and radiation. The benefits of lower gravity means that your body does not have to work as hard to maintain itself in a controlled environment. If protected from radiation, future space dwellers would have if properly vetted for disease and physical frailty would have the opportunity for a longer life span. But as mentioned in the answer before mine , these people would have to deal with irreversible physiological changes as the body adapted to its new environment. Isaac Asimov, science fiction writer and scientist speculated that after an extended time in lower gravity a person would in effect find himself/herself an exile from the planet of their origin. They would most likely only be able to return to the earth for only limited periods of time after strenuous preparation building muscle mass. The loss of bone material would make these people suceptible to serious injury from the simplest of mishaps. I would believe that you would age at the same rate in space as you would on the earth however , in a clean controlled environment , you would live longer. For example if you lived in space 100 years and you had a double living on earth 100 years , both of you would have aged the same amount of time, but having lived in space without the vagaries of gravity and disease, you would appear younger. Now I know this is contrary to the opinion of theoretical physicists concerning the passage of time , but the stated time differential is tied somehow to travelling at speeds faster than light, which I am sure that we will not achieve that for many years to come.