Look at the fleet number - the first two digits refer to the production year. F.A this one is no. 0315, so a 2003: http://www.bus-bild.de/1024/man-niederflurbus-2-generation-auf-95606.jpg My claim is supported by the fact that the now retired number 0018, so a 2000, had the "Bus of the year '99" sticker in it.

That applies also to Busses in Berlin. BVG gave their busses consecutive numbers, while the two first numbers identify type and production year.

However this is much more complicated than in Braunschweig. Here is an example:

3099 = 2003 (prototype, designated internally as 2004)
3101 - 3203 = 2005
3220 - 3322 = 2007

And so on.