Aruani is a planet that possesses a fifth of the gravity of the Earth. Humanity migrated to this planet 6,000 years ago and terraformed it to make its atmosphere suitable for human life. Would it affect the way they get older in some way? Would they look younger?
Answer provided by google.
(Apparently, there are lots of experiments, but not all have a straight conclusion yet.)
Earlier Space experiments had indicated that young male Drosophila flies exposed to microgravity showed an acceleration in aging. [...] By repeatedly video-recording, we have found a very marked increase in the locomotor activity of the fruitflies in Space. The males showed an accelerated aging response upon recovery, both in terms of physiological vitality assays (mating and negative geotaxis) and of life-span curves. [...] Drosophila females also increased their locomotor activity but did not show differential changes in the life-span curves.
Space Aging is an investigation aimed to study of the effects of space flight on the aging of C. elegans roundworm, a model organism for a range of biological studies. Microgravity causes a number of physiological changes, like heart and bone deconditioning, involving mechanisms that are poorly understood and may affect the rate at which organisms and astronauts age. Space Aging grows millimeter-long C. elegans roundworms in microgravity and compares their health and longevity with control specimens kept on Earth.
When astronaut Scott Kelly started the process of readapting to Earth's gravity after returning from a year on the International Space Station — the longest ever for a NASA astronaut — most of the biological changes he experienced in space quickly returned to normal. Yet some changes persisted after six months: most notably that 7 percent of his DNA remained alien "space DNA," NASA reported last week.
Not only that, Scott Kelly's telomeres — the caps at the end of DNA strands that protect the chromosomes and deteriorate with age — lengthened, and he came back 2 in. taller, although both his telomeres and his height reverted to normal proportions back on Earth.
For an immune cell, microgravity mimics aging
Telling someone to "act your age" is another way of asking him or her to behave better. Age, however, does not always bring improvements. Certain cells of the immune system tend to misbehave with age, leaving the elderly more vulnerable to illness. Because these cells are known to misbehave similarly during spaceflight, researchers are studying the effects of microgravity on immune cells to better understand how our immune systems change as we age.
Space is no trip to the spa -- astronauts experience dry, itchy, thinning skin. For this investigation, researchers measured skin hydration, trans-epidermal water loss, elasticity, and the fine structure on predetermined skin areas before, during, and after flight.
Apart from itching and dryness of the skin (possibly partly due to the special skin care being used in the ISS), a thinning of the skin and increased sensitivity combined with delayed healing of wounds and also an increased tendency to skin infections have been reported after a long stay in space.
Note: I'm not sure the skin changes are all due to microgravity; some of these effects could be a result of the air inside the ISS.
Also this, but it's too long to quote from: https://www.tandfonline.com/doi/pdf/10.1080/12265071.2000.9647549
Note that all these experiments were conducted in microgravity, not sure where 1/5G would fall on the scale in regard of biological development. Also, some of the accelerated ageing was observed after leaving microgravity and returning to 1G.
So my conclusion is that if they can make that telomere lengthening permanent then it could be conceivable that they would end up living longer, but first they might need to overcome the degenerative effects which would shorten their life expectancy.