On the 6th day God created cows, monkeys, dogs , human and various other creatures. This is a story about just those 4 creatures mentioned above, telling about the rest will be quite impossible to fit in here.
When God created cows, He told the cows "you will labor hard to plow the fields and some of you will be milked dry. I will bless you with 50 years of life" The cows replied "since our life are going to be so hard please kindly give us just half of it and make it 25 years" and God agreed.
When God created monkeys, He told the monkeys "you will hang from trees to trees and enjoy their fruits. I will bless you with 35 years of life" The monkeys replied "since our life are going to be so meaningless please take back 20 years and kindly give us just 15 years" and God agreed.
When God created dogs, He told the dogs "you will sit around guarding houses and bark at those who come or passes by. I will bless you with 20 years of life" The dogs replied "since our life are going to be so boring, 10 years will be enough. So please take back the remaining 10 years " and God agreed.
Then God created man. He told the man "you will be protected, you'll be well provided and you will party and be merry. I will bless you with 25 years of life" The man replied "wow thats really great, please give me the 25 years that the cows gave back, the 20 years that the monkeys refused and the 10 years that the dogs didn't want. I want to live for 80 years." and God agreed.
And this is why from birth till the age of 25, men usually live a great life of partying and was well protected and well provided for.
From the age of 26 to 50, men labor hard and some get milked dry by their family/relatives/peers.
From the age of 51 till 70 they hang out at their kids' houses and try to enjoy the fruits of their labor.
From the age of 71 till 80 they sit around and often 'bark' at people who come or passes by.