That's an interesting further distinction regarding field command.
Think 'Extra Billy' Smith (born 1797) would trump George Greene (born 1801) on this score, as the oldest field commander.
'Extra Billy' was promoted to B-G on Jan. 31, 1863, and led a Brigade at Chancellorsville, then at Gettysburg, before resigning on July 10, 1863.
Interestingly, the two oldest opposing Generals on the field at Gettysburg, were Union B-G George Greene and Confederate B-G 'Extra Billy' Smith, who both fought against each other at Culp's Hill, on July 3.