1st do you have proof Mary L is 2 in 1860? Everything and it isn't much shows her being born in 1860 Why the 1860 IA census? Do share if you know something we all don't.
1870 Census:
Daniel Slane 30
Catherine Slane 33
Mary Slane 4
Margaret M Sane 1
William Holt 15
Wade H Holt 12
Mary L Holt 10

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Wade Sr Age matches the 1860 census 26 years
If born in 1834
*Married 1852 = abt 18
*IA Census 1856 = abt 22 Louisiana Birth State
*AR Census 1860 = abt 26 Louisiana Birth State
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Mary Catherine we do have more info on
*Birth 25 Mar 1835 PA
*Fed IA Census 12 Sep 1850 = 15 PA listed 16,someone can not add
*IA Census No date 1856 = 22 PA listed 20
*AR Census 10 Jul 1860 = 24 PA listed 22
*Fed Mo Census 20 Jul 1870 = 35 PA listed 33
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They Moved from Iowa apparently to Arkansas to Missouri. Both boys in Iowa 53 & 58 and Mary L in 1860.
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If they were left with family apparently not her parents or they left them off the census. I haven't found parents for Wade SR, I just know Louisiana appears to be his birth state.
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I have decided to try and link all the Holts in Louisiana into family groups on FINDAGRAVE for a few decades before and after his birth. Wade Hampton SR is an ENIGMA to say the least.
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Some day his riddle will be solved. I thought another grand aunt and their youngest had died back in 1930 when the last we saw of him was 1. He passed away last year at 73 and found his children we will find what happened to him and his mother during those years, count on it.