The age of majority in Mississippi is 21 (Mississippi Code § 1-3-27)
You have to go by what your mother says. Other states may treat you as an adult. In Mississippi your contracts (like for an apartment) need to be ratified by an adult at age 18.
Phone a local lawyer in MS to check on more details.
While the legal age of majority in Mississippi is 21, a minor may petition the chancery court to be emancipated -- that is, to be considered an adult in the eyes of the law. Certain other conditions apply, depending on the situation.