Excerpt
Despite this well-reported figure of 1 to 3 years of age (or more correctly 1 to 3 years of antigen exposure), some pets do not show signs of allergies until midlife or later. Still others show signs as early as a few months of age, barely starting their puppyhood or kittenhood.

Many allergic pets have seasonal signs, showing itching only during the season when the specific allergens to which they are allergic are most prominent. For example, pets with allergies to Bermuda grass often begin showing allergic signs in the spring when the grass begins to awaken after the winter. Eventually, over many years, most allergic pets will develop signs that last all year long.