I agree with the other answers, Plus looks correct in the right.
If it's alright, I wanted to give some thoughts on the design you posted and the positioning of elements.
In my opinion as the Fields (Adult, Infant, etc.) are quite close to each other there might be a minimum "confusion" with the plus minus signs. I suggest to center the text with the number input to reinforce its relation (as it's already being done with the white color).
Also if the button is disabled I would display the sign with less opacity, the same way it's done with the background color.
Just a last comment, I guess the distribution of the inputs follow some order, if not I might expect it to be from less to more age which might help to understand quickly, before looking at the ages, the difference between infant and children (I am not english native and it is not as obvious to me).