The massive hot blue stars are the first to exhaust their fuel supply and leave the main sequence. As the cluster ages, more and more massive stars leave the main sequence leaving only the cooler low-mass stars. Ordering stars by their temperature, which places them in order of their mass, will show which stars will be the next to leave the main sequence. These stars are said to lie at the turnoff point . The age of the cluster is equal to the main-sequence lifetime of a star at the turnoff point.
Modern computers can calculated the lifetime of a star of a given mass with good accuracy.
A star like the Sun requires 10 billion years to exhaust its fuel. The cluster must have been around long enough for a sun-like star to run out of fuel.