But we do not have an instrument that directly measures age.
We don't have an instrument that "directly measures" silicon, or oxygen, or iron in a rock either. That hardly keeps some smart people from figuring out ways to measure them.
Of course there's no age-meter for a rock. It's nice, though, that there are direct counts of age for trees, or lake bottoms, or glaciers. All of those lead to an earth way over the 6000 years YEC's like.