George Paterson of Castle Huntly

I'm currently preparing for a trip to Scotland with my Dad and my older sister. I've been trying to figure out when the Robertsons left Castle Huntly to go and work for McDonnel of Glengarry. As I was looking around I came across a portrait by Sir Henry Raeburn of George Paterson, Robert Robertson's employer at Castle Huntly in Longforgan. So I thought I'd just park it here for now!

George Paterson died in 1817 - so it maybe that his death prompted Robert Robertson to apply for a new position. The last Robertson child to be born at Longforgan (according to census records) was Thomas who was born in 1820. The next child, George was born in Airlie about 1822 - so the family had definitely moved from Longforgan by then. That would make William around 10 when they moved however he gave his age as about 7.