The prerequisite for all four bus driving licences is first of all the possession of a category B driving licence. In addition, you must be at least 21 years old for D1/D1E bus driving licences and at least 24 years old for D/DE bus driving licences. Other age limits apply during and after certain vocational training courses. You can get more detailed information from your training company or driving school.
Driving licences can basically be divided into two categories: the actual driving licence for the vehicle in question (D1: "small" buses, D: "large" buses) and the associated trailer driving licence (D1E, DE).
D1 is a bus with a maximum of 16 seats, D is a bus with more seats.