He was 23 years old when he was enrolled (6).
He was living in Philadelphia when he was enrolled (6).
He was enrolled and mustered into service as assistant surgeon on 1 August 1862, near Washington, DC (1, 4, 6). He served as an assistant surgeon (1, 4).
Sinex claimed, "[H]e bore a good character both as regards his personal qualities and professional ones. He was always so far as my knowledge went very attentive to his duties and ever ready to attend to all calls upon him." (3).
He left on sick leave on 11 December 1862 (3). He did not return when his leave expired (3).
He was dismissed because of absence without leave, by general order 28, paragraph 1 (1, 4, 5, 6), either on 5 March 1865 (1; 4 has 5 March, but the year is illegible, 6), or on 14 May 1863 (2).
1 Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233. (In the roster)
3 letter, Sinex to Vincent, 2 May 1864
5 consolidated morning report, 91st PA, 19 March 1863 (Asst Surg Young)
6 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 5 May 2004 (John Young)
[I did not find an entry for him in the index on <www.ancestry.com> to the 1890 veterans' census, looking for a John Young rank surgeon]