I have seen a military enlistment for a Robert Andrews. He joined the Duke of Cornwall light Infantry 31st March 1916 in White City. He gave his age as 18 years 1 month and place of birth as Chelsea. He was discharged 16th June for giving a false age. Did not give any details of family but gave his fathers address as 1 Bedford Cottage, Bedford, Terrace, Chelsea.
The Robert James Cowan enlisted with the Royal Irish Rifles first embarkation was October 1915 when your Robert would have been just 17 years. Young men did lie about their age but he would have needed to have been 19 years to have been sent overseas.
Ray