Robin, like my colleague I don't see a William and Sarah Tucker in the 1850 census as living in Sumter County or nearby counties.
You had noted that Company D of the 10th Georgia Infantry had signed up at Americus, Georgia which was located in Sumter County, Georgia. I might tend to disagree with that as Company D of the 10th Georgia Infantry was formed in Richmond County, Georgia which would be in the Augusta area a long way from Sumter County. Although their was no requirements that a soldier had to be from Richmond County to be enrolled in Company D.
I think that the William M. Tucker who enlisted in October of 1862 in Bibb County, (Macon area) in the 10th Battalion Georgia Infantry would not have been your ancestor. If your ancestor had been born in 1805 I rather doubt because of his age he is the one your looking for. While it's true there were older men in the Confederate armies this was not the norm.
Company D of the 10th Battalion Georgia Infantry did have many soldiers from Bibb County, Georgia.
The service records for William M. Tucker of the 10th Battalion Georgia Infantry indicate that he was in the hospital frequently and was noted as absence without leave at least once. While the hospital visits would have been consistent with a man of his age what does not make sense to me is that he could have opted out of the service because of his age and received a discharge. This is not to say the notation of absence without leave may have been an error as sometimes many such notations were.
The first Confederate Conscription Act was on April 16, 1862 and required men from the ages of 18 to 35 years old. This William M. Tucker of Company D of the 10th Battalion Georgia Infantry appears to have compiled with the draft in October 1862. Because he was born in 1805 he would not have had to comply with the draft.
There was also a Stephen Tucker from Bibb County who also enlisted in Company D of the 10th Battalion Georgia Infantry perhaps some relation.
I saw nothing in the 1864 census for William M. Tucker and likewise did not see a Sarah Tucker whose husband had been in either the 10th Georgia Infantry or the 10th Battalion Georgia Infantry as receiving a Confederate widows pension.
Hope this helps