Stars in clusters like this are estimated to be of the same, or similiar ages, due to the fact that they're located spacially near to each other, usually embedded in the remains of the same gas cloud, have the same metallicity, and have co-moving velocities, ie. they travel as a group. This implies that they were born from the same interstellar cloud at roughly the same time.
The Sun is actually located on the outskirts of the remains of an open cluster of stars right now, or seems to be. Several of the stars in the Big Dipper, and a number of other stars scattered about the sky, appear to be a nearby, co-moving group, albeit more scattered than a young open cluster. It's called the
Ursa Major moving group.