Jesus was speaking privately to His disciples. He was not speaking to other people then living and He certainly was not speaking to people living in the 21st Century.
1 And Jesus went out, and departed from the temple: and His disciplescame to him for to shew him thebuildings of the temple.
2 And Jesus said unto them, See ye not all these things? verily I say unto you, There shall not be left here one stone upon another, that shall not be thrown down.
3 And as he sat upon the mount of Olives, the disciples came unto him privately, saying, Tell us, when shall these things be? and what shall be the sign of thy coming, and of the end of the world?