Its not that they cant leave its that they dont. During the Time of OoT, a great evil is lurking about. My guess is that the Kokiri are a peaceful race, almost pacifist. But they will fight... evidence that suggest this are the Kokiri Sword, the training ground, and the Deku Shield.

In the forest the Great Deku tree can protect them, once they leave the forest they are out of his reach. So when they leave they are on their own to defend themselfs. So they simply choose to stay in the forest, even though they are free at anytime to leave.

Also the Koroks are not the exact same Kokiri from OoT... Simple reason is because they have different names.

But yes, they are Kokiri we see in OoT, probably a different generation.

As for being immortal. They can die and be injured. If not then what would they have to fear about being attacked by the Deku Scrubs and such.(go to Kokiri forest before you defeat the Forest Temple)

My guess is that they either have a very long life, or they live until they are killed.

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[original looks better, but the site wont allow its awsomeness, so i had to shrink it a bit]