Is this a sentence fragment?
Closed to visitors since the 1960s, the prehistoric cave paintings in Lascaux, France, are between fifteen thousand and seventeen thousand years old.

 - no
 - yes
This is not a sentence fragment. It is a complete sentence because it expresses a complete thought.
Closed to visitors since the 1960 s, the prehistoric cave paintings in Lascaux, France, are between fifteen thousand and seventeen thousand years old.