Over the past 5 yrs, developed new allergies (asthma and milk intolerance) (29 y/o, F). I was wondering why its coming up in adulthood & prevalence?

Food Allergies. Many theories. Good question. May be in preparation and manufactuing techniques. For ex, dry roasting instead of boiling or frying peanuts increases their allergenicity. There is also cross-contamination of allergens during manufacturing. May be how and when food is introduced. Studies are underway. This is similar to the question of why autism is increasing. 4% of adults have food allergy.
May not be allergy. True allergies can develop at any time, however, what you are experiencing may not be an actual allergy but rather an intolerance. There is testing available to diagnose allergies. There are tests as well that can diagnose certain conditions such as lactose intolerance, but usually avoidance of offending agents and use of a lactase supplement can eliminate symptoms.