I've not seen a lot of research about how allergies develop as you get older.
For me, as a kid: very, very allergic to cats, kinda allergic to many food items and a little to horse hair (only noticable when shedding in the spring)
As a young adult: Only 2-3 food allergies remain, cats still strong, hayfever starts.
Then I took some shots against the hayfever for 2-3 years, and the cat thing has mostly improved and the hayfever is basically gone. So only 2-3 food items remain.
As an adult I developed an allergic contact dermatitis reaction to some sulfates (sodium lauryl sulfate definitely, sodium laureth sulfate definitely, and something in raw onion juice) after a bad burn on one of my fingers. Probably due to exposure while it was healing, since it's in a lot of soaps like Dawn have one or more of the two. Self-testing to find a soap that didn't blister my hand and then to narrow down which ingredients caused the reaction was a long & unpleasant process. So it's definitely possible to develop new allergies as an adult, as well as to lose existing allergies.
For me, as a kid: very, very allergic to cats, kinda allergic to many food items and a little to horse hair (only noticable when shedding in the spring)
As a young adult: Only 2-3 food allergies remain, cats still strong, hayfever starts.
Then I took some shots against the hayfever for 2-3 years, and the cat thing has mostly improved and the hayfever is basically gone. So only 2-3 food items remain.