The antonyms for the word "firstborns" are younger siblings, last-borns, and middle children. These individuals are born later in a family's life and are often more laid-back, independent, and not as focused on the achievements of their older siblings. In some cases, younger siblings may feel pressure to live up to the success of their firstborn siblings, but in general, they can carve their own path and create their successes. Last-borns, in particular, have been found to be more sociable, outgoing, and creative than firstborns, while middle children have a reputation for being more diplomatic, peacemaking, and adaptable.