After watching Catch and Release, I’ve come to realize something.
Peridot acts much more immature, when in a situation she’s not used to - she’s frightened, she’s aggressive, she loses her social skills. Some have interpreted this as her being childlike, similar to Amethyst’s seemingly “younger” personality in the flashback episodes. But this doesn’t match her previous portrayals, or her mature body type - she’s in no way coded as something that should be treated like a child.
So why does she act like that? Why did Amethyst act like that back in the day? Why would gems act less mature than they usually are? Well…
… Look at how they walk.
Look at how they act.
Look at them when they’re taken off-guard, or frightened, or excited.
Gems aren’t childlike.
All of them, in different ways.
They snap their jaws to show aggression, they show an animalistic combination of caution and curiosity when dealing with new objects and places, they even run on all fours when distracted. Fear makes them not just aggressive, but outright vicious and murderous, like mindless attack dogs. They occasionally have feline and canine physical traits, like fangs and glowing eyes. Younger!Amethyst literally went running after a thrown stick.
Amethyst, Peridot and Jasper, all presumably Kindergarten gems, seem to be the most prone to this, while Pearl, Ruby, Sapphire and Rose appear to be more civilized, likely as a result of having been regular Homeworld citizens trying to survive in a very uptight, controlled society for a long time. They’re used to restraining themselves from acting too savage. Lapis has an animalistic mindset ruled by emotion and impulsiveness, guarded by a civilized facade, reflecting her powers and element. She always seems more mentally present, even when upset, because she almost always has the upper hand and thus doesn’t feel the panic of a cornered animal.
Try rewatching an episode with the mindset of “these are extremely intelligent cats and dogs who were taught human-like standards of social etiquette.” It solves a surprising amount of questions regarding their occasional lack of maturity/manners/empathy/etc., unifying the traits normally associated with different human age groups.
Among them, why Peridot turns into a squeaky, slap-y menace when frightened. She’s got the survival instincts of a wild animal, with none of the strength. She’s like a housecat.
Tl;dr: gems have instincts and traits similar to felines/canines. Moments of immaturity happen when a gem hasn’t developed the social skills to deal with a situation in a civilized manner (due to fear, excitement, anger, lack of self-discipline, etc.), causing them to act like wild animals - which is often misinterpreted as “childlike” because we’re not used to seeing adults react in such an animated, uncontrolled way.