Methuselah lived one hundred and eighty-seven years, and became the father of Lamech. (Genesis 5:25)Lamech was born about 874 years after the creation of the world (3130 BC according to James Ussher's timeline). Lamech was Adam's great-great-great-great-great-great-grandson. But amazingly, at Lamech's birth, Adam was still alive— at an amazing 874 years of age, Adam would remain alive for over another half-century.
Although the number of humans on the planet would have quickly grown over the first 874 years of history, it seems reasonable that Lamech likely met Adam at some point in his life. It's amazing that hundreds and hundreds of years after the fall, Lamech was still likely able to hear a first-hand account of the fall!
Prior to the birth of Lamech, Scripture records of two men who were murdered (Cain killed Abel and the polygamous Lamech also murdered a man). While it seems (very) likely that other men were murdered in the first 874 years of the world, the reality of natural death (see Natural Death) was probably not yet fully realized.
When Lamech was born, all of the men in his lineage were still alive (his father was still alive, his grandfather was still alive, his great grandfather was still alive, etc). The first natural death recorded of in Scripture did not occur until 56 years after Lamech's birth.
Even more significantly, when Lamech was born, Enoch (see Enoch: A Prophet) was likely in the middle of his earthly ministry. Although the world was filled with wickedness, some hope could be found. It was into this world that Lamech was born— a world that had heard the voice of a prophet— a world that had some hope. A world that was vaguely familiar with death, but blind to the wickedness of sin.
State of the World (2948 BC): Noah's Birth - Genesis 5:28
Natural Death - Genesis 5:4-5
Enoch: A Prophet in the Pre-Flood World - Genesis 5:21-24
Methuselah: The Oldest Person (969 Short Years) - Genesis 5:25-27
Genealogy Timeline - Genesis 5