Yes Chamberlain was a young man when first a two star general.
But these photos are often made at the reunions of the union army.
Camberlain died in 1914 and this image looks quite elderly; this would not be possible as a real Brady image. Brady's Washington studio closed in 1881, New York in 1875, Putting this image before 1875, but his images were copied by Antony co. and reproduced for many years.