ESTIMATED AT vs ESTIMATED TO BE

As per the GMAT the correct usage by thumb rule is "ESTIMATED TO BE"

See this example:
"The fossils of x are estimated to be y years old...."


But there is another twist to it-
"estimated at" should be used only when the author is trying to indicate a location at which the estimation was made.


So, the conclusion is-


The rock that the Z-team found yesterday was estimated at the silicon valley by A-team earlier.


The rock that the Z-team found is estimated to be 5000 years old.