So, to answer the question, if "age of majority" is used only in the sense of an individual being capable to make rational, informed decisions about his/her own life, without having to answer to a parent or guardian, then perhaps it should be raised to 21. But this could come with its own set of issues; for instance, students who go to directly from high school to college at 18, and live off campus in an apartment. Would they be able to open bank accounts, sign a lease, or set up bills in their own names if they aren't considered adults?
Age of majority is frequently confused with similar concept, the age of licence, which also pertains to the threshold of adulthood but in a much broader and more abstract way. As a legal term of art, "licence" means "permission," and it can implicate a legally enforceable right or privilege. Thus, an age of licence is an age at which one has legal permission from government to do something. The age of majority, on the other hand, is legal recognition that one has grown into an adult. One does not need legal permission to grow up; it just happens.
For example, in any jurisdiction, the age at which an individual is allowed to exercise the franchise (vote), leave school without taking a diploma, enter into legally binding contracts (other than for necessaries, to which no age of licence applies), operate a motor vehicle, purchase and consume alcoholic beverages, and so on – these are all ages of licence, at which the law permits an individual to perform certain acts and exercise certain rights, with or without any restrictions.
Age of majority pertains solely to the acquisition of control over one’s person, decisions and actions, and the correlative termination of the legal authority and responsibility of the parents (or guardian(s), in lieu of parents) over the child’s persons and affairs generally.
Many ages of licence are correlated to the age of majority, but they are nonetheless legally distinct concepts. One need not have attained the age of majority to have permission to exercise certain rights and responsibilities. Some ages of licence are actually higher than the age of majority. For example, the age of licence to purchase alcoholic beverages is 21 in all U.S. States, but Mississippi is the only state which still retains 21 as the age of majority.