Thank you for your replies.
That throws a bit of a spanner in the work.
My grandfather was definitely in the Labour Corps and was 37 when the war started. When I said too young I meant for the year not his age. Unfortunately I don't have his army records and I'm not sure if he was in the army before hand, he appears on both the 1901 and 1911 census as a civilian.
However, I know his younger brother was definitely in the Royal Welsh Fusiliers from 1898 to 1914. He was born in 1880.