I've found a couple of potential birthplaces and years for John Roe that place him in Dorset and makes him a minor. The two below are the only John Roes I could find born in Dorset between 1738 (17 in 1755) and 1732 (13 in 1755)
John Roe b.1740 Corfe Castle.
Father: Charles Roe
Mother: Susana Roe
John Roe b.1738 Blandford Forum. Father: Thomas Roe
My only question is: is it likely that he was married to his first with for 19 years without having any children before marrying his second wife with whom he had 4?