Looks like Ervine was born about 1830, based on census records.
In 1850, an Irvin Masterson was living with mother Rachel Masterson in Harrison, Spencer, Indiana. As your Ervine married Margaret in Spencer County in 1866, I believe this is the same man. (1850 United States Federal Census: Year: 1850; Census Place: Harrison, Spencer, Indiana; Roll: M432_172; Page: 109A; Image: 608)