It is honestly uncertain whether or not Kanao is older than Aoi in Demon Slayer. We know that Kanao Tsuyuri is around 16 years old when we meet her in the main manga and anime series. However, Aoi’s age is unclear. Based on the fact that she became a slayer before Kanao, though, seems to tell us that she is likely older than Kanao, by a year or two perhaps.