Before I get to the statements on this issue that have been stated by 
more "official" spokesmen than myself, The first thought that I had when I had 
read this question, was that people always have their free agency. 
 
The first example I had thought of was Jonah and the city of Ninevah. (Jonah Chapter 3) 
He had prophesied that the city would be destroyed (vs. 4), but later was angry 
because the LORD didn't destroy it as he prophesied, because they had repented. 
 
The Lord will never take away our choice to choose, whether that choice be good or bad. 
 
Now here are some more substantial answers than mine above. 
 
President Joseph Fielding Smith wrote: “There have 
been various interpretations of the meaning of 
a generation. It is held by some that a generation is 
one hundred years; by others that it is one hundred 
and twenty years; by others that a generation as 
expressed in this and other scriptures has reference 
to a period of time which is indefinite. The Savior 
said: ‘An evil and adulterous generation seeketh after 
a sign.’ This did not have reference to a period of 
years, but to a period of wickedness. A generation 
may mean the time of this present dispensation.” 
(Church History and Modern Revelation, 1:337.) 
 
The Lord was speaking of the temple in Zion, that is, 
Jackson County, Missouri. The Lord later excused the 
Saints from building that temple because the mobs 
prevented it (see D&C 124:49–51) and because the 
Saints at that time had not kept the commandments 
as they should (see D&C 105:1–9). 
The day will come, however, when the holy city of 
God will be established in Jackson County, Missouri, 
and the temple will be filled with the glory of God as 
foreseen by the prophets. 
 
P.S. Look up those Scripture references on lds.org, for the D&C ones. The answer there comes straight from the LORD.