Stages in life

People say there are many stages in the evolution of a human life. You can say that it starts with being born-the Baby of the family, then you go to the stage of Toddler, then infant, Pre teen, teenager, Young adult, adult yet being young and then middle age and then old age. This the people along with the society have broken down in case of human evolution. I just believe that there are only three stages and that are childhood, Youth and old age, all the others are just to break things down more easily for all people to help them decide which age to blame for which problems. You see a child is a child whether it is a baby, toddler, infant, preteen or a teenager as according to law, anybody under eighteen is considered a child (well nineteen is technically not considered a child since it is lumped into being a teen but they are adult according to the law). Why do people do that is the question? I believe they want to attribute certain habits formed in a human in different stages of their lives and it is easier to do it that than to say that that child did it because people will say he/she is not a child but a young man/woman like maybe thirteen or fourteen years of age for example. If you say a child did it then it is assumed automatically that the child must be below the age of 10 or maybe even 11 or 12 to stretch a little bit. Nobody is going to say a young man of the age of 15 or 16 is a child since now a day they are being treated as adults when it comes to criminal activities.