Our learn by age two to say "No, Thank you!" in a cheerful voice, if it's something they don't want to eat. Well, except for my son, but he didn't say any words until he was three. By 3.5, he was saying, "No, thank you!" very easily.
I think it depends on family expectations, but ours have been raised from infancy that "no, thank you" or "none for me, thanks" are reasonable, and "yuck" or other variants are not tolerated.
But, we also have a family thing about not needing to swallow a test bite; they'll try anything, because they know it's fine to excuse themselves (with a quiet "Excuse me for a minute") and go spit an unwanted test bite into a non-visible garbage can or toilet. And then respond to additional portions with "No, thank you."