They aren't younger than Sanskrit, I am not sure where you got that idea from. As for the clustering, this is the Linguistic Center of Balance Principle . Essentially the idea is that if people migrated out from a location you're going to find the languages and cultures that came out of these peoples in greater proximity together nearer to the homeland because people will be splitting off along the path of migration. Thus India is unlikely to be the origin point of the Indo-European languages as there's only the Indo-Aryan branch present, representing only the strand of the migrations whilst it's likely to be out near the Black Sea because the closer you get to that area the more frequent and closer together the Indo-European language branches get.

People are more likely to stop at a shorter distance and settle down, and the others from their group just keep going, so the shorter distance is more densely populated with the languages descended from the original group than the other way around.