Note this implies quite a lot of capacities, that indeed also take some 20 years of education to fully acquire.

And more precisely - and also see self, personality, ego, role - every sane adult human being in a human society is treated as if one is a person: as if he or she has a self, a history, beliefs, desires, experiences, roles, responsibilities, duties, freedoms, rights, personal ends and interests, and the ability to reason about himself or herself at least as if all this is so, and as if there resides inside or connected to one's body a unique entity that has all these properties, that has a free will, and has minimally adequate ideas about society and reality, and that can be held responsible for his or her actions, and judged by others, and be punished or rewarded.

To put what matters a little more precisely and distinctly - and compare

"Every man has three characters - that which he exhibits, that which has, and that which he thinks he has."   (Alphonse Karr)

1.  Every person has a sense of self, and this comes in three guises:
    A. as what one is ......................... the self
    B. as what one believes one is ........ the ego
    C. as one pretends one is .............. the personality

2.  The self is what one has made of oneself, and comes from one's capacities, and decisions to - believe to - be a so and so, and to desire such and such ends.

3.  Personality and ego depend on the self.

4.  The self can only be experienced in part: There is more to anyone than anyone can experience at any time.

5.  The self changes gradually, as it depends on learning.

That every person (if not insane or very extra-ordinary) has a sense of self is simply a matter of everyday experience, in the sense that one experiences one's self (or believes one does) and others act, talk and behave as if they experience their selves.

The self is far more comprehensive that the roles it plays, so to speak: 

What one believes oneself to be is one of the things the self does - basically, constructing a theory about itself, and adopting that, and one's self is created and built during one's life, and is normally far larger and comprehensive than one can be conscious of or show at any particular moment.

It is important to see that, at least in ordinary reality, people make themselves to a considerable extent - as is quite clearly possible if what they believe they are, is, like their other beliefs, a theory, which is continuously revised and updated, and depends on their own choices and preferences.

And it is also important to see that one can only experience part of what one is at any time, and, moreover, that what one does experience is always in part effect of and in part representation of whatever caused the experience, and never the real thing, insofar as it is not a simple bodily pain or pleasure. (And even that is in fact a message of the kind "your toe requires attention").

It is also important to see that, in the terms of this remark, most people mistake their personality or ego for their self; that every adult who is not thoroughly insane plays some role (father, mother, employee, Good Christian etc.) nearly all the time, and tends to confuse himself or herself with the roles one plays in society; and that very few adults dare to act, think or feel out of the characters they familiarly play. (This last fact, which may be named the lack of individual character, is one of the root causes of human history being by and large a "record of the crimes and follies of mankind" (Gibbon): Mass-murdering, genocides etc. are perpetrated by perfectly ordinary and average people on perfectly ordinary average people for perfectly ordinary and average human weaknesses. If ordinary human beings would be able to create a just society, they would have done so long ago. See: Coster and ordinary men).