I found no straight answers for this until now.
Some scientist studied animals' life cycle and have determined a clear answers to the life cycle of animals.
However in humans I got all mixed answers.
Mostly I heard its degrading or shortening as lifestyles promote bad diet and lack of excercise.
The range could be as short as 50 years old to as long as 90 years old.
In the Bible some records show Moses lived for as long as 600 years old! Earlier records even show 900 years old man.
But today man lived as short as 50 years old for those who do not consider proper health. Some reached 90 for people who considers good living and good diet.
Mans age really doesn't have a clear and concise measurement of his life cycle. it is all varied unlike animals.
The only thing that is clear is that humans seems to live a little shorter for each generation. WIth some exceptions.