During the 19th century, I don't believe there were any international guidelines defining who could be seized as a prisoner of war. AFAIK, up to and including WW II, civilians were captured and imprisoned as POWs, at least by the AXIS powers.
The following are two different dispatches from Grant to Sheridan dated the same day during August 1864. In the first dispatch Grant orders Sheridan to seize as hostages, the families of Mosby's men and send them to "Fort McHenry, or some secure place."
The second dispatch orders Sheridan to send a cavalry division to Loudon County to "to destroy and carry off the crops, animals, negroes, and all men under fifty years of age capable of bearing arms." Grant claims that "[a]ll male citizens under fifty can fairly be held as prisoners of war, and not as citizen prisoners."
City Point. August 16, 1864—1.30 p. m.
{(Received 6.30 a. m, 17th.)
Major-General Sheridan,
Commanding, &c., Winchester, Va.:
Fitz Lee's division is,not in the Valley. We took quite a number of prisoners from it yesterday north of the James. Kershaw's division has gone to the Valley, and probably two brigades of Wilcox's division. Some cavalry has gone, but I do not know whose. I would not advise an attack on Early in an intrenched position, but would watch him closely with the cavalry, and if he attempts to move north follow him. The 100-days' men will have to be discharged at the expiration of their time unless there is a pressing necessity for detaining them for a few days on account of immediate active hostilities. The families of most of Mosby's men are known, and can be collected. I think they should be taken and kept at Fort McHenry, or some secure place, as hostages for the good conduct of Mosby and his men. Where any of Mosby's men are caught hang them without trial.
U. S. GRANT,
Lieutenant- General.
O.R. Series I, Vol. XLIII, pg. 811
City Point. August 16, 1864—3.30 p. m.
(Received 8 p. m. 18th.)
Major-General Sheridan,
Commanding, &c., Winchester, Va.:
If you can possibly spare a division of cavalry, send them through Loudoun County, to destroy and carry off the crops, animals, negroes, and all men under fifty years of age capable of bearing arms. *In this way you will get many of Mosby's men. All male citizens under fifty can fairly be held as prisoners of war, and not as citizen prisoners. If not already soldiers, they will be made so the moment the rebel army gets hold of them.
U. S. GRANT,
Lieutenant- General.
Ibid.