• young comparative younger, superlativeyoungest
• 1 a young person, plant, or animal has not lived for very long:
• a young child
• He's younger than me.
• You're too young to get married.
• young trees
• When I was young , I wanted to be a model.
• John was a great footballer in his younger days (=when he was younger).
• 2 a young country, organization, or type of science has existed for only a short time:
• At that time, America was still a young nation.
• Psychology is a young science.
• 3

   young lady/man

  spoken used to speak to a girl or boy when you are angry with them:
• Now, you listen to me, young man!
• 4 seeming or looking younger than you are [= youthful]:
• Val is incredibly young for her age.
• 5

   young at heart

  thinking and behaving as if you were young, even though you are old
• 6

   65/82/97 etc years young

  spoken used humorously to give the age of an old person who seems or feels much younger:
• Next week, Bessie will be 84 years young.
• 7 designed or intended for young people:
• I'm looking for something in a younger style.
• WORD FOCUS: young 
• other words meaning young: little/small used to describe young children, especially below the age of 6
• teenage used to describe someone who is between 13 and 19
• adolescent used to describe someone who is developing into an adult
• youthful looking young
• a young person: kid informal a young person
• teenager someone who is between 13 and 19
• youth a man or boy between 15 and 25, especially one who is involved in fighting or crime
• adolescent a young person who is developing into an adult
• minor a young person who is not yet legally an adult
• the time when someone is young: childhood the time when you are a child
• youth the time when you are no longer a child but you are still young
• adolescence the time when you develop into an adult

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.