The term Merchant Navy refers to a nations commercial shipping and crews. During the First World War, merchant ships were requisitioned to act as transports, hospital ships and cargo carriers.
The Merchant Navy was not the same as the army where there was an age requirement. Someone born in 1902 could have been 'apprenticed' to a merchant ship as a 14 year old. Yes, he may have lied about his age, if he looked older than he was.
Have you looked for his ships records to see what date he first appears and at what age?