lastborn vs young adult

lastborn

noun
  • The youngest member of any company or organization. 

  • The youngest child of a family. 

adj
  • Born last in a family; youngest in a the family. 

  • Younger than anyone else in a group. 

young adult

noun
  • A person who has achieved the age of majority but whose character and personality are still developing as they gain experience. 

How often have the words lastborn and young adult occurred in a corpus of books? (source: Google Ngram Viewer )