My boy has soy allergy. It'd trigger off bad rashes on his face and around lips.
On the safe side, feed homecooked food even when outside for longer, we did so up to 2 years old.
Now he's 32 months old, still no soy for him. I'd try again when he's 3 years old. If still allergic, try again another 6-12 months. Paed said many children outgrow allergies at about 5 years old. Though some would never outgrow them.
And with 1 allergy, it's possible the child has other allergies too, so be careful when introducing allergenic foods. My boy gets fungal infection when eating mushrooms, eczema also triggered by strawberries. So we're delaying introducing chocolate (this is an allergen!) to 3 years old and nuts to 4 years old.