This is mostly gameplay concession, but the simple explanation is that you are small and light and most adults are heavier an bigger so they are at no risk of mutating, unless the stuff their face 24/7. They also have access to anti-mutation herbs and potions from Tadeas and other alchemists so food mutation is not really a concern for them. There are other sourcess of mutation that could affect even adults.
As for the last part, I don't want to spoil this since this will be a major plot point in the game, but yes. Main character is different from other people on the island in more ways than one.