Geralt hasn't stuck around a human for this long before, so at first he thinks nothing of it when Jaskier comes back to him year after year, looking just the same. And when he doesn't die even after he must've passed sixty in age, Geralt chalks it up to the fact that any human would live past the expected when protected by a Witcher. He does get a little suspicious when Jaskier still looks the same after a century on the path together.