His parents moved the family to Auckland when Caldwell was a child and he was educated at King's College and Wanganui Collegiate School.

[1] On the outbreak of hostilities in August 1914, Caldwell attempted to enlist in the New Zealand Expeditionary Force, raised for service in the war, but was declined.

The citation, published in The London Gazette, read: For conspicuous gallantry and devotion to duty when leading offensive patrols.

He has personally destroyed five hostile machines, and has had over fifty contests in the air, in all of which he has displayed splendid skill and fearlessness, and has set an excellent example to his squadron.In October 1917 Caldwell was posted back to England as an instructor.

[7] Caldwell was awarded the Distinguished Flying Cross in December, the citation stating: "A fine fighting airman of courage and determination.

74 Squadron, Sydney Carlin, nursing his crippled aircraft to ground level before climbing out of the cockpit and jumping clear as it crashed.

[6] One of the squadron's flight commanders was flying ace Mick Mannock, and Caldwell thought highly of his tactical skills when engaging opposing aircraft.

He dived half a dozen times at the machine, spraying bullets at the pilot and observer, who were still showing signs of life ... On being questioned as to his wild behaviour after we had landed, he heatedly replied, 'The swines are better dead—no prisoners for me!'".

[1] Transferred to the Unemployed List of the Royal Air Force (RAF) on 17 July 1919, Caldwell returned to New Zealand in August.

However, the TAF lacked its own aircraft and relied on the PAF, soon to be renamed the Royal New Zealand Air Force (RNZAF), for equipment and groundcrew.

[12] During the Second World War, Caldwell served in the RNZAF, as station commander at Woodbourne near Blenheim and later Wigram at Christchurch, before being posted to India in 1944 and England in 1945, where he was promoted to acting air commodore, achieving full rank in 1946.

He retained an interest in military aviation and in 1960, along with Ronald Bannerman and Leonard Isitt, established the New Zealand 1914–1918 Airmen's Association.