American Samoa: territory since 1900; after the end of the Second Samoan Civil War, the Samoan Islands were divided into two regions. The U.S. took control of the eastern half of the islands. In 1900, the Treaty of Cession of Tutuila took effect. The Manuʻa islands became part of American Samoa in 1904, and Swains Island became part of American Samoa in 1925. Congress ratified American Samoa's treaties in 1929. For 51 years, the U.S. Navy controlled the territory. American Samoa is locally self-governing under a constitution last revised in 1967.The first elected governor of American Samoa was in 1977, and the first non-voting member of Congress was in 1981. People born in American Samoa are U.S. nationals, but not U.S. citizens. American Samoa is technically unorganized, and its main island is Tutuila.

Guam: territory since 1899, acquired at the end of the Spanish–American War. Guam is the home of Naval Base Guam and Andersen Air Force Base. It was organized under the Guam Organic Act of 1950, which granted U.S. citizenship to Guamanians and gave Guam a local government. In 1968, the act was amended to permit the election of a governor.

Northern Mariana Islands: A commonwealth since 1986, the Northern Mariana Islands together with Guam were part of the Spanish Empire until 1899 when the Northern Marianas were sold to the German Empire after the Spanish–American War. Beginning in 1919, they were administered by Japan as a League of Nations mandate until the islands were captured by the United States in the Battle of Saipan and Battle of Tinian (June–August 1944) and the surrender of Aguigan (September 1945) during World War II. They became part of the United Nations Trust Territory of the Pacific Islands (TTPI) in 1947, administered by the United States as U.N. trustee. The other constituents of the TTPI were Palau, the Federated States of Micronesia and the Marshall Islands. Following failed efforts in the 1950s and 1960s to reunify Guam and the Northern Marianas, a covenant to establish the Northern Mariana Islands as a commonwealth in political union with the United States was negotiated by representatives of both political bodies; it was approved by Northern Mariana Islands voters in 1975, and came into force on March 24, 1976. In accordance with the covenant, the Northern Mariana Islands constitution partially took effect on January 9, 1978, and became fully effective on November 4, 1986. In 1986, the Northern Mariana Islands formally left U.N. trusteeship. The abbreviations "CNMI" and "NMI" are both used in the commonwealth. Most residents in the Northern Mariana Islands live on Saipan, the main island.

Puerto Rico: unincorporated territory since 1899; Puerto Rico was acquired at the end of the Spanish–American War, and has been a U.S. commonwealth since 1952. Since 1917, Puerto Ricans have been granted U.S. citizenship. Puerto Rico was organized under the Puerto Rico Federal Relations Act of 1950 (Public Law 600). In November 2008, a U.S. District Court judge ruled that a series of Congressional actions have had the cumulative effect of changing Puerto Rico's status from unincorporated to incorporated. The issue is proceeding through the courts, however, and the U.S. government still refers to Puerto Rico as unincorporated. A Puerto Rican attorney has called the island "semi-sovereign". Puerto Rico has a statehood movement, whose goal is to make the territory the 51st state. See also Political status of Puerto Rico.

U.S. Virgin Islands: purchased by the U.S. from Denmark in 1917 and organized under the Revised Organic Act of the Virgin Islands in 1954. U.S. citizenship was granted in 1927. The main islands are Saint Thomas, Saint John and Saint Croix.
Given this text, what are the oldest and newest major territories of the United States?
The United States has five major territories. Guam, American Samoa, Northern Mariana Islands, Puerto Rico, and U.S. Virgin Islands. Of those, the two oldest are Guam, and Puerto Rico; both being acquired in 1899 at the end of the Spanish - American War. The Northern Mariana Islands is the newest of the five major territories, joining the United States in 1986.