Korra is about 60 years after the series with Aang. While other Avatars have lived much longer than that it gets reasoned that because Aang spent 100 years in suspended animation it cut down his life expectancy.

In addition to "The Avatar" being an ever changing role going from Air to Water to Earth to Fire then back to Air tribe, the timeskip means the overall world changes from more isolationist and tribal cultures to a "unified" 'Union City' where most of Korra takes place alongside steampunk tech because 60 years ago someone invented the hot air balloon. This eventually includes a giant steampunk robot and laser gun in the last season because why the fuck not? 🙄