Age of Adulthood

Adulthood , the period in the human lifespan in which full physical and intellectual maturity have been attained. Adulthood is commonly thought of as beginning at age 20 or 21 years. Middle age, commencing at about 40 years, is followed by old age at about 60 years. 
Physically, early and middle adulthood are marked by slow, gradual declines in body functioning, which accelerate as old age is reached. The muscle mass continues to increase through the mid-20s, thereafter gradually decreasing. The skeletal mass increases until age 30 or so, and then begins to decrease, first in the central skeleton (pelvis and spine) and last in the peripheral skeleton (fingers and toes). Throughout adulthood there is a progressive deposition of cholesterol in the arteries, and the heart muscle eventually grows weaker even in the absence of detectable disease. The production of both male and female hormones also diminishes with age, though this cannot be directly related to the gradual diminution in sexual activity that occurs in both males and females between 20 and 60.
There is clear evidence that with increasing age adults display a slow, very gradual tendency toward decreasing speed of response in the execution of intellectual (and physical) tasks. Slowing rates of electrical activity in the older adult brain have been linked to the slowing of behaviour itself. This decline in the rate of central nervous system processing  does not necessarily imply similar changes in learning, memory, or other intellectual functions. The learning capacity of young adults is superior to that of older adults, as is their ability to organize new information in terms of its content or meaning. Older adults, on the other hand, are equal or superior to young adults in their capacity to retain general information and in their accumulated cultural knowledge. 

Advantages:-

You can do things and sign up for things that you couldn't as a kid.

You no longer have to go to school.

You have more money to spend on whatever you want.

You can attempt to achieve your goals and dreams.

You can buy cigarettes and go to bars.

You can vote.

You can live almost anywhere you want.

Disadvantages:- 

You are expected to be more mature and you have more responsibilities. (Ex: You will be shamed for acting like a kid or you would go to prison for having drugs instead of getting a slap on the wrist, like you did as a teenager)

You no longer can go to school. (You become less intelligent and your chances of meeting people and making friends goes way down)

You need a job to live.

You have less options for deciding what you want to do with your life.

You no longer have anyone giving you advice or telling you what you should do.

You have less free time.

ADULTHOOD  IS LIKE LOOKING BOTH WAYS BEFORE YOU CROSS THE STREET AND THEN GETTING HIT BY A AIRPLANE.