A popular fan theory as of late is that Peridot is actually really young. She did not participate in the gem war, since she didn't recognize the Crystal Gems and doesn't seem to know who Rose Quartz is. Not only that, but she seems to be figuring out all the abilities she possesses on the fly, often surprised when her gadgets work. Read more about the theory at the source below, and give your own input on the 'Peridot is totally a baby gem' theory!
Source
Twitter: Emerald