Allergies can develop at any age, from the first few months of life through late adulthood. Most allergies first appear in childhood, with nearly 30% of children in the U.S. diagnosed with at least one allergic condition by 2024. But new allergies also emerge in adults, sometimes in people who never had allergic reactions before. The timing depends on the type of allergy, your genetics, and what your immune system has been exposed to.
How Your Body Becomes Allergic
An allergy doesn’t happen the first time you encounter a substance. It takes at least two exposures. During the first encounter, your immune system quietly “learns” the allergen. Immune cells process it, present it to other cells, and trigger the production of a specific type of antibody called IgE. These IgE antibodies then attach themselves to mast cells, which are found throughout your skin, airways, and gut. At this point, you feel nothing. You’re now sensitized but not yet allergic.
The second time you encounter that allergen, it binds to the IgE antibodies already waiting on your mast cells. This cross-linking triggers the mast cells to release histamine and other inflammatory chemicals, producing the symptoms you recognize: sneezing, hives, swelling, itching, or in severe cases, anaphylaxis. This whole process can happen within minutes of re-exposure. The gap between first exposure and sensitization, then between sensitization and a full allergic reaction, is why allergies seem to appear “out of nowhere.” Your body was quietly priming itself before you ever noticed a symptom.
Food Allergies in Infancy and Early Childhood
Food allergies tend to show up earliest. Cow’s milk allergy often appears in the first year of life, since milk-based formula or dairy is typically one of the first foods an infant encounters. Egg allergy follows a similar timeline. Peanut allergy usually surfaces between ages 1 and 3, when children are first introduced to peanut-containing foods.
About 3.9% of children ages 0 to 5 have a diagnosed food allergy. That number rises with age: 5.0% in children ages 6 to 11 and 6.9% in those ages 12 to 17. This increasing trend partly reflects cumulative exposure to new foods over time and partly reflects the fact that some food allergies persist while new ones continue to appear.
Current guidelines from the American Academy of Pediatrics recommend introducing major allergens like peanut and cooked egg around 4 to 6 months of age, regardless of family history. This is a reversal from older advice that told parents to delay these foods. The landmark LEAP trial found that introducing peanut to high-risk infants between 4 and 6 months reduced peanut allergy rates by over 80% compared to waiting until after 12 months. A large Israeli birth cohort study found similar results for cow’s milk: infants who received cow’s milk formula in the first two weeks of life had significantly lower rates of milk allergy by age 3 to 5.
Seasonal and Environmental Allergies
Hay fever and other environmental allergies, such as reactions to pollen, dust mites, and mold, typically develop later than food allergies. Children rarely show symptoms of seasonal allergies before age 2, and the condition becomes much more common after age 6. In 2024 data, only 11.6% of children ages 0 to 5 had a seasonal allergy, compared to 25.0% of children ages 6 to 11 and 24.0% of those ages 12 to 17.
This makes sense biologically. Environmental allergies require repeated seasonal exposures over multiple years before sensitization builds to the point of producing symptoms. A child needs to live through several pollen seasons before their immune system produces enough IgE antibodies to trigger noticeable reactions. Research tracking birth cohorts has found that allergic rhinitis (the clinical name for hay fever) often first appears around age 6 or 7.
Eczema and the Allergic March
Eczema (atopic dermatitis) is often the very first sign of an allergic tendency. It affects roughly 12 to 14% of children across all age groups, and it frequently appears in the first year of life. Pediatricians watch for eczema closely because it can signal a higher risk of developing food allergies, hay fever, and asthma later on.
This progression is sometimes called the “allergic march”: eczema in infancy, followed by food allergies in early childhood, then asthma and hay fever in school-age years. Not every child follows this sequence, but severe eczema in infancy is one of the strongest predictors that other allergic conditions will develop.
Why Some Children Are Protected
Your likelihood of developing allergies is shaped heavily by early life. Children with older siblings have lower rates of hay fever and asthma, an observation first documented in a study following over 17,000 British children. Attending daycare in the first six months of life also appears protective against eczema and asthma. Growing up on a farm, especially with exposure to cowsheds, reduces the risk of allergic diseases, and the effect is even stronger if the mother had that exposure during pregnancy.
The underlying idea is that early and diverse microbial exposure trains the immune system to tolerate harmless substances rather than overreact to them. Infants with less diverse gut bacteria at one month of age are more likely to develop eczema by age 2 and hay fever by age 6. Children born by cesarean section and those without siblings tend to acquire gut bacteria more slowly, which may partly explain their higher allergy rates.
Adult-Onset Allergies
New allergies in adulthood are real and surprisingly common. Food allergies are actually more likely to emerge for the first time in adulthood than seasonal allergies are. Shellfish is the most common adult-onset food allergen by a wide margin, followed by milk, wheat, tree nuts, and soy. A 2018 survey of more than 40,000 adults found that many people with food allergies first developed them well past childhood.
One pattern that helps explain adult-onset food allergies is infrequent exposure. The less often you eat a particular food, the more likely your immune system may eventually misidentify it as a threat. Shellfish fits this profile perfectly: most people eat it only a few times a year, giving the immune system sporadic, unpredictable encounters rather than the regular exposure that seems to promote tolerance.
Relocating to a new region can also trigger environmental allergies in adults. Moving somewhere with different pollen types means your immune system encounters novel allergens for the first time. After a few seasons of exposure, sensitization can build to the point where you develop hay fever symptoms in your 30s or 40s despite never having them before.
Hormones and Allergy Timing
Hormonal shifts play an underappreciated role in when allergies surface or worsen. Before puberty, boys are more likely than girls to have allergic conditions. After puberty, that pattern flips, and women become disproportionately affected. This shift points to the role of estrogen, which can act directly on mast cells through estrogen receptors, making them more reactive.
Women often notice that their allergy and asthma symptoms fluctuate with their menstrual cycle, worsen during pregnancy, or change at menopause. Estrogen and progesterone influence how aggressively mast cells release histamine, which means hormonal transitions can unmask allergies that were previously subclinical or make existing allergies noticeably worse. Oral contraceptives and hormone replacement therapy can have similar effects. For women who develop allergies around puberty, pregnancy, or menopause, hormonal changes are a likely contributing factor.
The Role of Genetics
Having one parent with allergies roughly doubles a child’s risk of developing allergic conditions. Having two allergic parents raises the risk further. But genetics alone don’t determine whether allergies develop or when. The current understanding is that genes create a predisposition, and environmental factors determine whether that predisposition is activated. Two siblings with identical genetic risk can end up with very different allergic profiles depending on their microbial exposures, diet, birth method, and the timing of allergen introduction. This gene-environment interaction is why allergy development remains somewhat unpredictable, even in families with strong allergic histories.