D&C 84

 Concerning the New Jerusalem, the Lord revealed to the Prophet Joseph Smith - 

"Which city shall be built, beginning at the temple lot, which is appointed by the finger of the Lord, in the western boundaries of the State of Missouri, and dedicated by the hand of Joseph Smith, Jun., and others with whom the Lord was well pleased.

4 Verily this is the word of the Lord, that the city New Jerusalem shall be built by the gathering of the saints, beginning at this place, even the place of the temple, which temple shall be reared in this generation.

5 For verily this generation shall not all pass away until an house shall be built unto the Lord, and a cloud shall rest upon it, which cloud shall be even the glory of the Lord, which shall fill the house."  (Doctrine and Covenants 84:3–5)

"Generation" has many meanings.  One being - 

"...a type or class of objects usually developed from an earlier type..."

I personally believe that generation in this case is the Church, as it continues to grow and develop before the second coming of the Savior