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I see that the same question was asked at
https://talkingscot.com/forum/viewtopic.php?f=24&t=10322 16 years ago and no satisfactory answer was obtained.
Are these the letters you mention?
https://www.ayrshireroots.com/Towns/Mauchline/Letters%206.htm - if so it looks as if it is John Shields, not Andrew Young, who lived there. In a later letter there is mention of brother John having moved to the Byre of Loudon, and in 1841 there are indeed two John Shields, possibly father and son, at Loudon Byers. The elder one is listed as aged 65, which means that he was born between 1771 and 1776.
There's no listing of Wintock(s) in
https://scotlandsplaces.gov.uk/ which means either that it is a variant spelling or somewhere that no longer exists.
Could you give a little bit more information? For example, what were the names of Andrew's wives and when did he marry them? What were the names and birth years of his children? What were the names of his parents and when was he born?
Also a bit of context - if the above letters are not the ones you are referring to, could you post (an) extract(s) from the letters showing the part(s) where Andrew Young was mentioned? Is there a definite statement that he lived in the parish of Loudoun? Are the brothers and cousins his?
In the 1841 census there are just two Andrew Youngs in Loudoun - one aged 8, so obviously too young; the other, age recorded as 35, with Mary, 30 and two children.
In 1851 he is 40, with wife Mary, 36, and four children, all born in Loudoun and living in Darvel in the parish of Loudoun.