According to Pakistani nationality law:
s.14 Dual Citizenship or nationality not permitted. If you hold a citizenship outside of Pakistan the Pakistani citizenship is terminated
- Unless one decides to renounce the other
- Unless one has citizenship in Britain or its colonies
- Unless one is a female married to a man who is not a Pakistani citizen
s.14A Renunciation of citizenship. If a person pledges allegiance or becomes a citizen of another country they have forfeited their Pakistani citizenship
- The exception is if a child (under the age of 21) who does not have his Pakistani citizenship and would like to resume it after turning 21 is allowed to do so.
The above text is copied from Wikipedia but there are links to the actual text of the Pakistani Citizenship Act of 1951 which says the same thing.