This lad, at the age of 16, decided to join the Army as 18-yr old JACK MARTIN - and so is "mis-filed" in the WW1 Service Records (WO 363):

On 30 October 1905 he joined the RFA and served until dispersal in November 1919 (when he was transferred to the Army Reserve).
Declared age 18y 5m (born Deptford, Kent)
Parents: Frederick Steven & Agnes.
Brothers: Sidney, Bertram, William & Albert

Birth registered Q2 1889 (St Olave)
Regt No.: 39735 Royal Field Artillery
True name declared 1914.

Anc****y, MAR, pages 199270-199305