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.