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,
that once the age of one star is known, then they use it to determine
the ages of all other stars inside the clusters.

Well a new discovery is showing that new stars are being born inside
some of these clusters, in at least one cluster so far. It's possible
that it happens inside most if not all other clusters too. It's also
looking like no new gas from the outside is entering these clusters, all
of the star formation comes from gas expelled from dying and dead
members of the cluster, producing new stars.

Yousuf Khan

https://phys.org/news/2017-03-star-c...applecart.html