It is a general observation that mammals other than human beings grow up faster and achieve height and adulthood much earlier than a human child if life span is considered. Same is true in case of other vertebrates if their growth rate is compared with the growth rate of human body. Especially birds which can live several decades grow up to full height and reach puberty within months or a year or two.
Humans’ children remain dependent upon their parents for years. Parents feed them till they grow enough to earn for their own living. Whereas in case of mammals, new born stumbles few times after birth and then start walking along with her mother. Their dependency on mother or care from father is short term. Mammalian animals born with natural instincts. While human baby passes through several stages of body and brain development. A new born human child can neither talk like adults nor can perform his own basic tasks.
During the growing stage of a human kid, certain hormones are released which trigger the growth of different body parts. Any deficiency or abundance can lead to abnormality of some sort. But such cases are very rare where hormonal imbalance happens during growth. The rate of metabolism in humans and the rate at which growth hormones are secreted inside the human body are different from other mammals and vertebrates. These are induced by the genetic system of living organisms which is different among all species. When a human kid grows to a certain height and weight, some hormones are ceased from further release. For example, secretion of growth hormones stop or become very limited.
Brain is the master of whole body. Human brain is the most advanced than any brain of any other living being on this planet. Recent researches have proved that major brain development continues till late 40s. After that neurons still grow to store more data. A person keeps learning more or less during its whole life. Human experience and knowledge increases with advancing age. And this is only possible if brain has capability of further growth.
Archaeological evidences prove that height of past humans was different from what it is today. Several thousand years old fossils of giant humans have been discovered which are the evidence of growth variation during different eras of human occupation.
Historical records also reveal the age variation among different human generations. Age of earliest known human beings who ever ploughed the Earth to take benefit from its fertility appears to be of several centuries. Many religious and history books hold the strong evidence that earliest ancestors of human races had life span of centuries. Some records reveal that age of Adam was more than 900 years. This is the age a man can only dream of today.
Even though, human life is very short now, still, he marches towards more and more progress which has ultimately resulted into the progress of today which has no match at all. Today, human is much more intelligent than human of several thousands years ago.