If you have a habit you can break that habit. If it is in your personality to be cruel that is because you practice cruelty thus if you stop practicing it then you no longer have the personality trait of cruelty, thus you can change every aspect of yourself theoretically. Of course practically some traits are very deeply embedded and may transcend the people you choose to be. This you could suggest is what makes you, you! Maybe that is what sticks from birth to death, because I would find it hard to argue that have exactly the same personality all throughout life.
The goal of life is to become more aware and conscious of oneself and the world so it is possible to pick and choose aspects of one's personality that one wishes to deepen and those that one wishes to let recede into the background. That in itself is going to mean choosing the personality presented to the world.
Yes, in some instances as an individual grows up they can choose their own personality. You may not be able to change who you are as a person, but you can choose what another person sees or think they see in you. If an individual has a different outlook after an event, their personality can change so I can not see why a person could not change their personality at any given point or time.
Yes, as one grows up, he or she can choose their own personality, because they can see what is out there and decide for themselves. A person can see how people act and decide how they want to act. Just as we can choose what we eat, we can choose how we react to each other.
As one grows up one may wish to change their personality and can take active steps to do so. A success may be claimed if one can squelch the negative impulses they are trying to control and are able to develop the positive aspects and appearance they seek but ultimately the underlying personality remains the same.