I am an adult with severe dairy and nut allergies. I travel about once a month for work or pleasure and almost always on Delta, Jet Blue, or Southwest. I’m not airborne (thankfully) but I am contact sensitive (only hives so far from such reactions) for nuts particularly almonds.
I’ve been fortunate not to have any issues with being allowed to pre-board and always contact the airlines ahead of time to note it and have had some particularly notable experiences on Delta and Jet Blue.
Most recently when traveling for work from on Delta on January of 2019, from DCA to ATL, the flight attendants even made the effort to reorganize who was in which section so that the one who was a trained nurse was in my section. I’m planning on flying quite a bit this year so hopefully all experiences will be this positive.