Oh really?
Psalm 90:10 The years of our lives are 70; and if by reason of strength they be 80 years, yet most of them are labor and sorrow; for life is soon cut off and we fly away.
http://genforum.genealogy.com/dna/messages/129.html says that:
MUMMA Generation time span - So how long a time is a generation? Some geneticists and anthropologists suggest 15 to 25 years as the number of years per generation. I believe this substantially understates the value for modern times, i.e. in the last 500 years. First, this time appears to be the time for the "first birth" of females and not an average time for the birth all of her children. For example, if we simply assume a woman's child bearing period was typically from age 15 to 45, the median child would be born around 15 years after her marriage, suggesting an average generation time of 30 years, based on the mother's age.
Thus, with the shortened generational lifespan in Biblical times, Natural Law is quite correct.
I quote scripture and you quote genealogy? Let me repeat.
Matthew 1:17 Therefore all the generations from Abraham down to David are 14 generations; and from David down to the Babylonian captivity are 14 generations; and from the Babylonian captivity down to the anointed, are 14 generations.
Matthew is using the Psalm 90 definitions of Generation in order to tell a specific chronological time story. The first interval of that story is strictly historical, the interval from Abraham to David.
Using 70 as the length of a generation, the interval implied by Matthew is 14 * 70 = 980 years. Do some study on your own and stop listening to others. The math proves the 70 year generation.