Per 1900 census Adam immigrated in 1883 and was in the United States for 17 years and was a naturalized citizen.
The 1920 census also give 1883 as the immigration year.
1920 United States Federal Census
Name: Georg Koberstein
[George Koberstein]
Home in 1920: Philadelphia Ward 33, Philadelphia, Pennsylvania
Age: 26 years
Estimated Birth Year: abt 1894
Birthplace: Pennsylvania
Relation to Head of House: Head
Spouse's Name: Alice
Father's Birth Place: Germany
Mother's Birth Place: Germany
Marital Status: Married
Race: White
Sex: Male
Image: 1089
Household Members: Name Age
Georg Koberstein 26
Alice Koberstein 27
John Walker 27
Margaret Walker 23Social Security Death Index
Name: George Koberstein
SSN: 163-03-9992
Last Residence: New Jersey
Born: 5 Jul 1893
Died: May 1965
State (Year) SSN issued: Pennsylvania (Before 1951 )World War I Draft Registration Cards, 1917-1918
Name: George J Koberstein
City: Philadelphia
County: Philadelphia
State: Pennsylvania
Birthplace: Pennsylvania
Birth Date: 5 Jul 1893
Race: Caucasian (White)
Roll: 1907754
DraftBoard: 31U.S. World War II Draft Registration Cards, 1942
Name: George James Koberstein
Birth Date: 5 Jul 1893
RESIDENCE: Philadelphia, Pennsylvania
Birth: Philadelphia, Pennsylvania
Roll: WW2_2240630
Note regarding the images for the states of PA, MD, WV, and DE. These four states were scanned at the National Archives facility in such a way that the back of one person’s draft card appears on the same image as the front of the next individual. The result is that when you click to view the original image, you will see the correct front side of the draft card, but the back of the previous soldier’s card. Ancestry is aware of this problem, and is working to correct this issue.