Today, Java has generics, which I haven't studied and therefore don't understand. I believe the consensus is that you can't really be a Java programmer without understanding generics, so -- oops! -- I'm not a Java programmer anymore. My knowledge is oh-so-2003.
Tomorrow Java may get closures -- the debate rages. If it does get closures, all the Java programmers will have to update their skills or fall off the wagon.
The other day I heard a guy on a podcast talk about his disastrous attempt to test-run Drupal 3.0 (god knows why). That software is about three or four years old. Apparently it doesn't work anymore. PHP and/or MySQL and/or Apache have evolved right out from under it.
The names of all the programming languages used today are at least 10 years old. Just as the name of the English language dates back to Chaucer's time and before. Have you tried reading Chaucer?