"Absolutely"??? You've lost me. Burnside was promoted to Major General on March 18th, 1862... he was 38 years old at that time.
Was he demoted, or ...?
That fella was much older than 38, methinks; and, although I do not recall seeing a post-Civil War picture of him, I cannot believe he ever got rid of the famous "Sideburns" that he made famous.
As I see it ... (not easy at the present time ... off subject), he retired with the rank of Major General.
I expect he did, but I would have to check if it was as a MG in the Regular Army, rather than as a volunteer. He started out as a Colonel in a Rhode Island volunteer regiment.
I wonder if a determination of the subject's age would be accurate, given the conditon/ quality of the photograph.
You wonderment is well founded. If I can isolate the URL for that image, I will send it to an old grad school friend who has published widely on uniforms. Perhaps he can provide some information.