"Virtue, then is a state of character concerned with choices, lying in mean, that is, the mean relative to us, this being determined by a rational principle, and by that principle by which the man of practical wisdom can determine it"
Aristotle maintains that virtues are always located between two vices, each vice either involves an excess or a defiance of what is required. For example, courage is the mean between foolhardy bravado on the one hand and cowardice on the other. Each vice involves either a deficiency (having too little) or excess (having too much) of what is required.
But a mean for one person is not a mean for another and depend entirely on the circumstance because what is appropriate for are situation may not be for another and a virtuous person is one that is phronesis (practical wisdom), the ability to determine what each situation needs.
Aristotle says this can't be attained by the young because they lack of life experience to be able to make complex decisions. Adult will require it if they have a good upbringing since you become virtues by doing virtuous deeds from an early age. It is the matter of acquiring those habits before one needs to know what they are for and can use them naturally in any circumstances once they are old enough to understand them.
Hinman puts it like this:
The childhood conception of morality usually comes from the parent, and is mostly negative (don't do that!) and rules and habit formation are central, but in the adult conception of morality would come from within and is usually positive, this is the kind of person that I would like to become' and is mainly virtue centred. The idea of Habit is crucial to Aristotle's ethics, since in Greek philosophy virtues are often associated with skills. Aristotle writes:
"We acquire virtues by doing virtuous acts. We acquire a skill by practising the activities involved in the skill. For example, we become builders by building and we...