Food allergies are one of the most common medical problems. They commonly develop early in childhood, and they less commonly develop later in life. In fact, food allergies that occur in childhood will sometimes get better as the person gets older. That being said, it is technically possible for allergies to food to develop at any point, so if your brother has symptoms consistent with allergies to peanuts, he should set up a visit to discuss this with his primary care doctor
One of the particular concerning things about peanut allergies is that they tend to be one of the more serious forms of food allergy. Therefore, if there is in any question, it would be best to avoid peanuts until he can be tested by his doctor and this problem ruled in or out.
He should also be on the lookout for any signs of a serious allergic reaction. These would include any swelling of the face, eyes, lips, or tongue, any nausea and vomiting
after ingesting peanuts, or any wheezing or trouble breathing. If he experiences any of these symptoms, this is a serious medical emergency and he would need to seek immediate medical attention in an emergency room.