Trichogramma wasps emerge from “their” egg already fully self-fertilized and ready to lay more eggs, but idk if that really counts since the first portion of their life cycle is consuming another insect’s egg as a parasite… (I know a ton about them because I released them in my house for years to combat pantry moths when I had birds - they do an absolutely spectacular job.)

It’s a species without males, due to a bacterial infection that suppresses males almost entirely. If treated with antibiotics they start to produce males again. Essentially, the lack of males means they have to be fully fertile immediately. But idk if that makes it sexually mature or not. I think that’s definitely an edge case either way, but it’s the closest I can think of.