Hello & Welcome.
Seems a bit old for James to have been born 1730 and then marry at age 45 in 1775 and have son Charles in 1786 (age 56). Of course it's possible ... not unheard of.
I can't seem to find any other children for James Robb and Isobel Carnegie except Charles 12/10/1786. Curious what years Mary , David , Ann and James were born. Wondering what is the year of their last child's birth?
Also , what year did James Robb die and what age does it say he was on the grave?
I see this James Robb grave but it doesn't mention the wife:
https://www.findagrave.com/memorial/235266627/james-robbWhen you zoom in on the stone you can see Charles there and Isobel...
I guess the biggest problem is the fact that there are 2 other James Robbs born at the time within a 25 mile radius. There is James Robb 11/10/1730 Fourdoun ...and then there is one in 1732 in Drumoak and one in 1731 at Cortatchy.
The thing about the Fourdon one is the father's name is David and from what you say James Robb also had a son named the same.
You probably saw this ?
David Robb
Scotland, Births and Baptisms, 1564-1950
Christening: Jan 22 1698 - Fordoun, Kincardine, Scotland
Father: David Robb
A line of David Robbs from Fourdon.