I know some people who were able to eat shellfish throughout childhood but then developed an allergy to it when they got older. explain why please?

More exposure: To develop an allergy, the immune system has to learn to recognize a substance (e.g. Shellfish). Each time you eat the food, there is a risk your immune system will finally recognize it. Most people develop seafood allergy later in life because it takes many exposures over many years before their immune system finally "wins the lottery", so to speak. We don't know why this only happens to some.