According to the official stats, something about 30 million are under 30 in Germany (total population 84 million)--and of that about 4.5 million are foreign born. So about 15 percent of the age bracket are foriegn...and we can estimate at least 95% of those are Middle Eastern or Turkish Moslems.
We know that wild compiracy theories, usually virulently anti-Semitic, are regularly printed and debated about in the Moslem world...so a probable guess is that 90% at least of those under 30 usually under-educated Islamic immigrants in Germany believe the conspiracy theories about 9/11.
This would skew the results a bit eh? This would mean that of native Germans under 30, really closer to 15% (not 31%) believe the conspiracy theories.... Convincing lunacy can always garner something over 10% (Perot got what,a vote of 19%?) so this doesn't have me too worried. I'd be willing to bet a number of those native Germans are from the East too--still very suspicious and naive of the West generally and the US in particular.
Germans better worry though about the effect of gullible undereducated immigrants on their population however.
Germany's demographics below:
http://www.kmike.com/country/gmdemog.htm Germany
Population: 83,029,536 (July 2001 est.)
Age structure: 0-14 years: 15.57% (male 6,635,328; female 6,289,994) 15-64 years: 67.82% (male 28,619,237; female 27,691,698) 65 years and over: 16.61% (male 5,336,664; female 8,456,615) (2001 est.)
Ethnic groups: German 91.5%, Turkish 2.4%, other 6.1% (made up largely of Serbo-Croatian, Italian, Russian, Greek, Polish, Spanish)
Religions: Protestant 38%, Roman Catholic 34%, Muslim 1.7%, unaffiliated or other 26.3%