I headcanon that Jasper is a BABY compared to the rest of the Gems. Not super young, but way younger than Pearl or Garnet. She was probably born in the Kindergarten, which is why in the battle scene during “Do it for Him/Her,” her weapon and outfit is extremely simplified. But she’s still older than Amethyst or Peridot.
Well, Jasper fought in the war. She fought against rose and possible with rose before rose rebelled. Jasper isn’t a baby compared to the rest but peridot is.