On Thursday, March 9, 2017 at 8:07:54 AM UTC-5, Yousuf Khan wrote:
One of the default assumptions about star clusters is that all of the
stars inside each of them are of nearly the same age as each other. So a
star cluster that is 10 billion years old, will have stars that are all
10 billion years old. This is such a well established astronomical rule, ...

It's more of an reasoned assumption than a rule. Given that there are a only a handful of younger stars in the cluster that was studied, it might be fairer to say that "a very tiny part of what we know about star clusters might be wrong."