First names vary with time, with some names becoming popular for a decade or more, and then may drop to near zero.
I'd be curious to know more about the popularity of German first names over the last 100 years or more.
A first step in this direction can be found at http://www.nordicnames.de/Aussprache.html
. Most names have numbers in brackets (for example: Adelheid [70-] )--which indicate how old people with this name are likely to be (your friend Adelheid is likely to be 70 or older).
A truly wonderful graphic interface for exploring first names can be found at http://babynamewizard.com/namevoyager/lnv0105.html
(applet requires Java-enabled browser). There is more here than at first meets the eye, so keep playing with it. Currently Name Voyager is limited to American first names over the last 125 years, but the FAQ gives information about how to provide names for other countries.
I had discussed once with someone about possibly providing German first names so they could generate a German version of it, and she thought that the Datenschutzgesetz prevented such data from being collected, or at least, divulged. Can this be true? What invasion of privacy could possibly result from knowing how many Adelheids were born in the last 10 years?
Also, even if the data were limited to, say, the 15th-19th century, I would find it fascinating to watch the patterns ebb and flow. So I hope someone can give some clues about where information about first names, especially in digitized form, might be found for German first names.
Oh, and enjoy the Name Voyager, but be warned, it's quite addictive!