At the time God determined to flood the earth, Methuselah, Lamech, and Noah were still living. God was gracious to Noah and his family, and as a result, Noah was chosen to "rescue creation" (Gen. 6:8). Lamech, who had killed a man, and who was the first man to marry more than one wife (Gen. 4:19-24), died five years before the Flood at the age of 777. Methuselah, about whom nothing is written concerning his character, died within a year of the Flood.
Noah was 600 when the floods came. There is some debate as to how long it took him and his three sons to build the ark, but it appears from Genesis 6:18, that his sons were old enough to marry and to assist Noah in building the ark (see http://www.answersingenesis.org/articles/2010/06/01/long-to-build-the-ark). Whether it took 120 years, 100 years, or approximately 75 years, it was long enough for the population of the earth to have ample warning of the pending judgment of God upon the earth (Heb. 11:7)! We find that during the time the ark was being constructed, the rest of humanity continued to ignore Noah's warnings and to live life as they always had (Mt. 24:37-38; Lk. 17:26-27).
Now, to my point. Jesus said that people would be just as they were in the time of Noah; they would be wicked (Gen. 6:5), and life would be going as it always had (Mt. 24:38). Just as Noah's "folly" had been a warning, today, born again believers are warning our lost world of God's pending judgment. And, like those who drowned, people scoff and ridicule us. That is why, when He comes, He will catch the world off-guard, as do thieves their victims (Mt. 24:29-51; Lk. 12:39; 1 Thes. 5:2; 2 Pet. 3:10; Rev. 3:3; 16:15).
Regardless of how the lost respond, we must continue to warn them until it is too late!