  In this Letter, we analyse the distributions of stellar ages in Giant
Molecular Clouds (GMCs) in spiral arms, inter-arm spurs, and at large galactic
radii, where the spiral arms are relatively weak. We use the results of
numerical simulations of galaxies, which follow the evolution of GMCs and
include star particles where star formation events occur. We find that GMCs in
spiral arms tend to have predominantly young (< 10 Myr) stars. By contrast,
clouds which are the remainders of spiral arm GMAs that have been sheared into
inter-arm GMCs, contain fewer young (< 10 Myr) stars, and more ~20 Myr stars.
We also show that clouds which form in the absence of spiral arms, due to local
gravitational and thermal instabilities, contain preferentially young stars. We
propose the age distributions of stars in GMCs will be a useful diagnostic to
test different cloud evolution scenarios, the origin of spiral arms, and the
success of numerical models of galactic star formation. We discuss the
implications of our results in the context of Galactic and extragalactic
molecular clouds.
