Can you newly be allergic later in life to some food which you've eaten frequently as a child?

Generally not. True food allergies typically don't develop as an adult. As the digestive tract ages, however, it sometimes gets finicky, and some foods that have been well tolerated begin to cause some discomfort. The difference is that an intolerance can never become life threatening, but the end result is the same - whether it's an intolerance or a true food allergy, all you can do is avoid it.
Yes. You can become allergic to foods at qany time without regard to previous exposure.