Fluctuating Age Disorder

Fluctuating age disorder is a genetic disorder found in races with a longer than average lifespan (1K+) and instead of ageing like other races but halting at ~26, people with FAD bounce between 2 different ages for their whole life. If one of the points is younger, their mental state too will revert, it's not just a physical thing and is generally considered a major pain to people that do have it as it can lead to multiple puberties. But it also means that people with FAD never age beyond their higher cap, meaning they're effectively immortal even if their race isn't otherwise.