I realize that this post is almost 18 years old but it shows up repeatedly so I am updating some of the information.
It appears that Robert Lusk b. 1710 was living in Chester County, Pennsylvania by 1742. There is a document dated 7 Feb 1742 in which Robert Lusk paid to take over a land warrant in Nantmeal Township, Chester County. It appears that Robert Lusk b. 1710 died in or before 1784 at which time his sons John Lusk b. 1760 and Robert Lusk b. 1761 sold the farm and moved to Washington County in south western Pennsylvania. John and Robert were born in Chester County.
The land that Robert b. 1761 bought in Peters Township, Washington County became part of Fayette Township, Allegheny County when Allegheny County was formed in 1788. The creek on which his land was located was likely Chartiers Creek. Chartiers Creek was a boundary of Fayette township. Robert is living in Fayette Township in the 1790 Federal Census and in the 1800 Pennsylvania list of taxpayers. In 1803 he is listed as paying taxes in North Beaver Township, Beaver County.
There is a newspaper death notice 28 FEB 1836 for Robert b.1761