You observed three different star clusters and found that the main-sequence turnoff stars in cluster 1 had spectral type A, the main-sequence turnoff stars in cluster 2 had spectral type B, and the main-sequence turnoff stars in cluster 3 had spectral type G. Which star cluster is the oldest

Age of given star cluster can be measured by the Main-sequence turnoff. star cluster is defined as the group of stars which are bounded with each other through gravitation. There are two types of star clusters:

Globular cluster which contain hundreds of thousands of stars which are very old in age.
Open cluster which contain less than few hundred of star which are very young in age.

Stars in the cluster remain at same distance and all the stars from the cluster are of same age. As the age of the cluster increases there is decrease in the mass of main-sequence turnoff stars. When a star reaches to main-sequence turnoff point then it enters into stellar evolution phase where they start converting hydrogen into helium and hence release tremendous amount of energy. After 5 billion years type G star began to evolve whereas, type B star began evolve after 1 billion year after which type A evolved hence Type G is the oldest star cluster as it is also very deficit in elements i.e. hydrogen.