Retention of Danish citizenship
If you were born abroad and have never lived in Denmark and have not resided in Denmark under circumstances that indicate solidarity with Denmark, you will lose your Danish citizenship upon reaching the age of 22, unless you become stateless as a result.
According to established practice, you are considered to have resided in Denmark if you have been registered in the Central Register of Persons (CPR) at a Danish residence for at least 3 consecutive months. According to established practice, you are also considered to have resided in Denmark if you can document that you have had a residence address in Denmark for at least 3 consecutive months, and the reasons for and circumstances surrounding the residence have been of a nature that indicate that the residence was intended from the start to be of at least 3 months’ duration. Finally, residence in another Nordic country for a total of at least 7 years can be equated with residence in Denmark.
It is a prerequisite for preservation that you can present documentation or otherwise confirmed information about residence in Denmark.
source: Ministry of Immigration and Integration