YEARS  AGO
The temples have been standing through every earthquake, every eruption, every flood and every war in history, (which began with the invention of writing.)

The Ggantija Temple on the island of Gozo is nearly six thousand years old!   It was already very ancient when the ring of stones that we recognize as Stonehenge was built two thousand years later.  Every famous building that you can think of was made later than the temples of Gozo and Malta.  There are ruins of earlier structures elsewhere in the world, but nothing that still has rooms and walls standing.

How do they know the age?  If archaeologists find undisturbed charcoal or other organic matter inside the ruins or under the stones, there are special tests to find out how old the building is by our reckoning of time.   Other tools for dating include comparing the style of pottery and other objects found inside. 

Stonehenge, England
 

Ggantija Temple, Gozo, Malta

 The temples were not all built at the same time.  They were rebuilt, expanded and improved
over hundreds of years as people learned new skills and wanted more space.