If we are looking into military history there are many pictures of interest from the period of the 1850's onwards. During the American Civil War Mathew Brady was especially productive and from this war date the first pictures of real action. Now, don't expect much of them as it wasn't possible to capture movement on the plate in these early photographs. We see however, the smoke hovering over the battlefield at Antietam and the bombardment of a fort. Much more of an impression make the pictures of the aftermath of the battles and those of the veterans.
Veterans of earlier wars are photographed as well and there even are some pictures of splendidly uniformed veterans of Napoleon's Grande Armee. A set of pictures depicting fifteen Napoleonic veterans can be found here. These soldiers have probably been photographed in 1858 on the 5th of May. On that date veterans donned their old uniforms and honored their general and emperor at the yearly anniversary of his death in Paris. So most of these men are in their late 60's or seventies.
They are however not the earliest born persons in photos, that honor goes to veterans of the American War of Independence.
Grenadier Sergeant Taria wears a bearskin and the uniform of the Grenadiere de la Garde of 1809-1815.
Monsieur Moret wears the uniform of a grand hussar of the 2nd Regiment, 1814/15. Red pants, green jacket and gold embroidery on the tassel.
M. Dreuse of the 2nd Light Horse Lancers of the Guard, circa 1813-14 wears a splendidly plumed shako.
Twelve more splendid and interesting photographs of the Anne S.K. Brown collection can be found here.