The next step up are true clusters, called open clusters . Almost all open clusters we see in the sky are relatively young. The youngest is about 1 or 2 million years old, while the oldest is about 6 or 7 billion years, making it just older than our own solar system. Most are younger than 100 million years old. We can tell the ages of the clusters by studying the stars in them.
Stars evolve, and astronomers have a pretty good handle on just how fast stars age.
So by looking at the stars in the cluster we can tell pretty well how old they are. Clusters of stars probably all form from the same collapsing gas cloud, much like how we believe the solar system formed. Once they form from the gas, the hot, bright stars burn and blow away the remaining gas in a process that, to be honest, is still not very well understood. Over time, the stars in the cluster pass near each other, and affect each other gravitationally. Some stars can slingshot out of the cluster that way.