Why is it important to work with young children and guide their behavior at an early age?

The "guide their behavior" part is questionable, but there are lots of studies showing that the first 3 years of a child's life are critical for their development. Their brains go through more development, and more setting up of connections between neurons in those years than at any other time, for one thing. They need exposure to quality language, and help in learning and developing their skills. For example, when toddlers start walking, parents often walk right behind, ready to catch them if they fall, but letting them practice their new sklll. That needs to be applied to everything their learning, whether physical or mental. 

EVERY child, at early ages, are like sponges - they just take everything in.  If a child is NEVER told what he or she should do or not - they will do what ever and never learn to be a good person.  Correcting bad behavior is VITAL.

of course to make them behave and think as you the parent wish them to

a child learns basic behavior and respect before the age of 7 (mostly 2 to 4 ) 

trying to teach respect to a 14 year old is almost impossible and you will see them self destruct and feel hopeless to help them 

If somebody does not do that for young children, getting them to observe the norms of good behaviour, to learn how to get along with other children and other people, then it is more likely that the youngsters will 'go off the rails' later, end up in court with criminal records and generally be a nuisance in society.

This is important so they grow up to be decent adults and not entitled man-children. 

Because they will just develop social skills.

cause they might learn better when theyre young