Older than Old

How old is old?  I mean for an object or a building; not a person.  In Minnesota, 150 years is old.  Heck, stuff from the 1970's is old.   Here is Sicily, stuff is older than that.  There are ruins and archeological digs where they have to ruin stuff from the 16th century to get to the stuff from the 15th century.  Today I visited two sites that were both BC.

We visited the remains of a fortified Greek castle from 400 BC.

Archimedes was involved in the design of this castle.  Archimedes was a mathematician and engineer.  He discovered the formula for pi and designed compound pulleys.

But this temple of Apollo is even older.  This is from the 6th century BC and is located right in downtown Syracuse.  This place is very old.