Jürgen Hubert said:
I dunno about the UK, but as for Germany he would theorethically automatically get German citizenship until the age of 23, whereupon he must choose which citizenship he must give up.
I had often heard that I would have to give up one of my citizenships at a certain age, but that never happened. I'm 28, and a dual US/German citizen, was born in Germany to a German Mother and U.S. father. At the age of 26 I renewed my German Passport and went to live in Germany for two years. I had no issues with both citizenships, even with all of the registration procedures I had to go through. The Bundeswehr even contacted me for my pflicht, and I told them I was a dual citizen ( I didn't have to serve, though I think that had more to do with my age, and that I was working rather than the citizenship).
On the other hand, I had a colleague in Germany who was of British/German descent, and she renounced her British citizenship at the age of twenty-one, because she thought it was required. It turned out that it wasn't, and there was some sort of mixup, and then for a while she wasn't a citizen of either country. It was complicated, and I don't remember all the details, but the point is, that the experiences vary from person to person, and even the government agencies don't know how to handle dual citizenships.
But as to the original question - one thing I do know is that german citizenship laws were changed in 1975. Prior to that, you qualified for citizenship if your father was german, after either parent could be german. There was an application process that could be gone through for children born to a german mother before 1975 to get them citizenship, my older brother had to do it that way.
I found a link :
http://www.germany-info.org/relaunch/info/consular_services/citizenship/generalinformation.html#q1
that's probably more accurate than my ramblings above
