It is very difficult to provide an accurate date for the portrait based on the photograph alone - my estimate is some time in the mid- to late 1860s, although it could also conceivably have been from the early 1870s. However, the reverse of the card mount enables us to narrow down the estimate considerably, as it has a 3c green George Washington Internal Revenue stamp.
These stamps were used to indicate that a tax had been paid on the photo. The taxes were levied by the United States Federal Government during the Civil War as a revenue raising exercise, and the system operated between 30 June 1864 and 1 August 1866. As was directed by the authorities, the photographer has "cancelled" the revenue stamp in ink with his initials, "J.L."
John Loeffler was a studio photographer and publisher of stereoviews who lived and worked on Staten Island, New York from the early 1860s until the early 1900s. He was born c. 1833 in Frankfurt am Main, Germany and emigrated to the United States in 1854. His sons August and Alexander Loeffler were marine photographers.