So, you need a closet shelf for sweaters?
That's much different than a book shelf! So the rules for those don't apply here. I have used 3/4" plywood up to 48" wide for a similar application. I wouldn't use it for a 55" wide shelf however unless it was reinforced with a steel 1 1/2" X 1/8" thick angle. It may not sag, but it won't look good if it should. A better material would be 3/4" or 1" thick hardwood like Oak. Plywood would have to run lengthwise, and that's not very stiff compared to the Oak. The width of the shelf may be up to 24" as in my case, so gluing two pieces together will be necessary. this will add strength because you can oppose the grain directions.
A lot depends on the manner of supporting the shelf at the rear. A continuous support will be better than just at the ends. A 1 1/2" or 2" wide Oak strip glued to the front edge will add stiffness and that's what I'd do.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)