Originally Posted by Aviatrix
This could well be a Danish rule rather than a US one. I know that in a number of European countries the law is that children may have dual nationality but adults must choose one or the other - I know this is the case in Germany where (with some exceptions) the age limit for dual nationality is 23, and I understand other countries have similar rules.
You have to distinguish between people who are born dual nationals, but are merely applying for a passport of the "other" country in an "old" age, and people who are actually applying for another nationality.
Denmark does not allow dual nationality as a rule. This means that you have to surrender your existing nationality if you want to
apply for Danish nationality, but Danish nationals can apply for other nationalities without losing Danish nationality, subject to the foreign country's rules. However if you are
born with dual nationality - eg. if you have parents of different nationalities - you keep Danish nationality for life, on condition that you are born in Denmark or - if born abroad - that you have lived at least one year in Denmark before you reach 22 years.