The Kokiri are children forever. They simply don't grow up beyond 7 or 8. Along with this restriction, they can't leave Kokiri forest. If they do, they die. None of them really seem to mind, though. They just go about their daily routine of standing around.
The Hero of Time (Who is probably the first Link) was raised around the Kokiri and believed he was one of them. As it turns out, his dying mother just left him with the Great Deku Tree for protection. As a result, Link grows up wearing Kokiri clothing: Green tunics and weird pointy caps. This is probably the reason why all future incarnations of Link wear the familiar green suit.
In a few hundred years, the Kokiri "evolve" or magically change forms to become Koroks. This is to better use magic to glide around from island to island and make new forests.