Originally Posted by PhlyingRPh
This is correct. At age eighteen an American citizen must choose between their foreign citizenship or that of the U.S.
Obviously, most people will choose to retain their U.S. citizenship. However, even if you do not carry a valid British passport, you will never lose your British citizenship. The expired passport can be used as evidence of your right to live and work in the E.U.
This is incorrect. I am 24 and a dual citizen holding US citizenship. I was never forced to choose. Neither were my two brothers. There are no laws in the US having this requirement. (And see my above post regarding Supreme Court cases upholding some peoples' rights to dual nationality.)
Germany
does have this requirement. In fact, the new laws introduced not too long ago actually state that if one does not before the age of 20 inform the authorities of one's intention to keep or relinquish Germany nationality (in favor of another), they will assume one has choosen to relinquish Germany nationality.