E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Born with a silver spoon in ones mouth. Born to luck and wealth. The allusion is to silver spoons given as prizes and at christenings. The lucky man is born with it in his mouth, and needs not stop to earn it.
One can see, young fellow, that you were born with a silver spoon in your mouth.Longmans Magazine, 1886.