I don't think that part of the scene was trying to imply that Nanaki's 'tribe' had three times the lifespan of humans, just that Nanaki was in the stage between being a child and an adult. After all, if you take the dialogue literally, Bugenhagen is a direct ancestor of Nanaki. My theory is that Nanaki's lifespan is exponentially higher than humans, so that his maturity is equal to a human a third of his age now, but that could increase to 10 times or more by the end of his life.