My youngest son hasn't had any since around age two. My other three had all of theirs. The reason he didn't, and he's now eight, is because of severe food allergies, especially to corn.
He developed really bad eczema around two, and when we had him tested it seemed as though he was allergic to almost all foods, including cats and dogs, and of course stuff like grass and pollen.
We've had to really restrict what he could eat over the years, making it seem almost impossible for him to eat most foods. Try finding foods that don't have corn or gluten in it. It seems that corn is in everything, including medicine in some form.
Now that he's eight, he's really starting to be able to tolerate more regular foods. He still suffers from eczema, although not nearly as bad as before.
I will say this, he's been the healthiest of all the kids. He's never had any ear infections, which is pretty common with younger kids, or strep throat, which one of the kids always seems to pick up every year while in school.
I'm not kidding when I tell you he's maybe been sick once or twice in eight years.