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Part of their regeneration process is that they keep growing taller and taller over the centuries; so as, younger ones are typically the size of a regular human being and indistinguishable. As their bodies grow throughout the years, the regeneration process allows them to live for millennia; however, even though they regenerate and rejuvenate, as their bodies grow their internal organs do not and eventually their body's size will outgrow the resilience of their internal organs and eventually they will cease functioning. This is old age to them.