Re: When does the human body reach full maturity?

Lisa,  The first items reaching maturity in the human body are the ear 
ossicles which finish their growth in-utero so that the baby can hear 
clearly before birth.  Some research indicates that ultrasound, at high 
energy levels, might be able to damage these tiny bones because bone 
absorbs that kind of energy more than soft tissue.  As far as other 
systems in the body, if you define maturity as being able to function, 
then almost every system reaches that stage before birth so that the baby 
can stay alive after parturition.  The switch from calling the unborn 
child an embryo to calling it a fetus is at about the end of the first 
trimester and that change in terminology is because at the end of the 10th 
to 11th week, in-utero, the baby has developed all the major body systems 
it will have and the work left for it is to develop to a functional (life-
supporting) level of maturity before birth.  So, we call the first 
trimester baby an embryo and after that we call it a fetus.  The only 
system that is not mature at birth is the reproductive system.  This 
system begins to develop in girls first and boys last.  In girls the 
reproductive system generally starts to develop (puberty) sometime between 
the age of 9 to about 12 and it takes approximately 2 to 3 years to become 
functional.  In boy puberty generally starts between the ages of 11 to 
about 15 with another 2 to 3 year to become fully functional.  In girls, 
the fact that the system is functional does not mean that it is at the 
optimal potential for producing a healthy newborn.  Some research 
indicates that babies born to teens have a higher risk of low birth 
weight, failure to thrive, and other problems all the way to mortality; 
the younger the mother the higher the risk.  So you see that maturity is a 
tricky question and this is why there is so little agreement in the 
literature.  When looking at puberty, for example, nutrition and exercise, 
as well as other things all have dramatic effects on commencement.  
Regular fasting or just low calorie intake (especially low fat 
consumption) will delay puberty.  The same results can be seen in 
individuals participating in Olympic-level type training.  I have probably 
raised more questions than I have answered but that is the way in science.

Here is another few sentences that I wrote for a question about the 
differences between boys and girls in the developmental stages that might 
give you some more ideas about maturity:  

     First of all boys and girls start developing differently almost from 
the very beginning of life.  About the 7th week of life in the uterus, if 
the embryo is going to be a boy, a hormone called testosterone will start 
to be synthesized and cause all the ducts that form the majority of the 
male genital (reproductive) track to start developing.  At the same time 
this male embryo will also produce a protein that inhibits development of 
the ducts that would form most of the female reproductive track.  Thus 
there has to be active stimulation of the male development and active 
inhibition of the female system.

     If the embryo is genetically set to become a girl, nothing happens 
during the 7th week because the male tubes have to have testosterone and 
that isn’t there.  The female ducts will start to develop on their own 
(They must be smarter than the male ducts, right?) at about the 11th week 
in the uterus and will grow into a normal baby girl at birth.  So the 
female system doesn’t need any signals or help in that way but just 
develops on it own.

     When an adolescent hits puberty there is another difference in 
development.  Girls have female sex steroids; probably most significant 
would be progesterone and 17 beta-estradiol.  Boys have male sex steroids; 
probably most significant would be testosterone and dihydrotestosterone.  
This difference causes girls to develop mammary glands, start having 
menstrual periods and begin storing fat deposits on their hips among other 
things. This difference causes boys to have more muscle mass, start 
growing facial hair, and begin having a lower voice among other things.