Depending on when you count the creation of X11 from (i.e. from W, or the first X11 release), it now 40 (or nearly 40) years old.
No other GUI system has lasted this long in general purpose production use. An X application written nearly 40 years ago, will still connect and work with the latest version of X, and will work as well as it did when it was first created.
X11 represents and incredibly technical legacy that still serves us today, even though it was created in a time where the field of computing was very different than it is now.