Since Mary A Arnett Young was found to be in Darke County Ohio for both 1850 and 1860 I would be inclined to think the marriage took place in this county.
The following Christopher Young is the closest match to your Christopher C Young assuming he was counted in the 1850 record. What is odd about this find is that there are two Arnett families living on either side of him. Christopher also owned his land in Darke county in 1860. Perhaps there is a record of the purchase and sale of this property on file which would give you a clue of his arrival in Darke county. The 1860 census shows only your Christopher and two other Young's with first initial C who were about the same age and the other two were not in Miami County in 1860.
There are 94 Young's indexed in Miami county in 1850 of which 8 are in Union including Christopher, Mary and Joseph. Perhaps they died and he then married Mary Arnett within the year.
Miami County joins Darke County to the east and Darke County is bordered by Indiana on the west. Union township in the southwestern portion of the county bording Darke County.
Darke County has a good genealogical website on USGENWEBhttp://www.dcoweb.org/marriage/
Anyway, worth checking into. Good Luck!
Philip Young, 60, b. PA
Mary Young, 33, b. OH
John Young 31, b. OH
Susannah Young, 28, b. OH
Lucinda Young, 7 mos, b. OH
3 Oct 1850 Union, Miami, Ohio
Page: 151 Roll: M432_711
Christopher Young, 24, laborer, born OH
Mary A Young, 19, born OH
Joseph Young, 1, born OH