All car seats used in Europe must have an ECE approval sticker to show that the seat has been tested and complies with the basic safety requirements as set out in European Safety Standards ECE R44/04 or R129/i-Size. R44/04 came out in 2005 and seats that meet R44/03 are also still legal to use. R44/03 car seats were produced from 1995 until 2005, so if you are using a car seat that has R44/03 on the sticker it could potentially be 23 years old, and should really be replaced.
Seats that were manufactured before 1995 and have been illegal since 2008.
Car seats bought in other countries which do not have a European approval sticker must not be used in Europe.