DD didn't have an allergy to dairy, "just" an intolerance, but she was about a year-and-a-half old before a reintroduction worked without ill effect. She's since developed a strong reaction to gnat bites and cat (and dog?) dander. Those don't look like they're going away anytime soon, and we now carry an epipen.
DS has a nightshade allergy, the result of gut damage at birth, and it came on when he was about 15 months old. He's now almost 30 months old and he's *just* been able to tolerate small amounts, after a year on probiotics and being strictly off all nightshades. He still reacts to small direct exposures, but not as severely, and he can now have foods with small amounts of paprika or "spices" in the ingredients without noticing a reaction, and I can finally eat nightshades and nurse him without a problem.
ETA: I think the cause of an allergy can be a factor. DD's cat reaction is inherited; dh is also allergic to cats and has been his entire life. So it's likely here to stay. DS's is a result of an injury, and as such, the doctors and dh and I have been assuming (hoping) it can be healed, like many injuries can.