According to a history of the U.S. Department of Veterans Affairs, following the Civil War pensions were only authorized to Soldiers that were discharged due to an illness or injury. Then in 1890, The Dependent Pension Act of 1890 broadened the requirements for pension to include a veteran that had served more than 3 months and was unable to conduct manual labor due to their service-related injuries or illnesses. It stated the following:
"That all persons who served ninety days or more in the military or naval service of the United States during the late war of the rebellion and who have been honorably discharged therefrom, and who are now or who may hereafter be suffering from a mental or physical disability of a permanent character, not the result of their own vicious habits, which incapacitates them from the performance of manual labor in such a degree as to render them unable to earn a support, shall, upon making due proof of the fact according to such rules and regulations as the Secretary of the Interior may provide, be placed upon the list of invalid pensioners of the United States, and be entitled to receive a pension not exceeding twelve dollars per month, and not less than six dollars per month, proportioned to the degree of inability to earn a support; and such pension shall commence from the date of the filing of the application in the Pension Office after the passage of this act, upon proof that the disability then existed, and shall continue during the existence of the same."
Spouses, dependent children and dependent parents were also allowed to file for the pension. In three years the number of Civil War pensioners rose by 500,000 and the total pay-out amount doubled. The Sherwood Act of 1912 broadened the scope for pensions once again, allowing all Union Soldiers a pension at age 62 regardless of any notable disability. Note that Confederate Soldiers were not authorized a federal pension, but would have instead filed for a pension within the state they lived in. These pensions were often only for indigent or disabled Soldiers.
Glasson, William Henry. History of military pension legislation in the United States, Columbia University, 1900, pg. 114
Confederate Pension Records, The National Archives, December 12, 2007