At what age and how do young Catholics know or be notified that they have committed a mortal sin or a venial sin? At what age and how do young Catholics know or be notified that they have committed a mortal sin or a venial sin? Basically, do they tell the priest at Confession that they have committed a mortal sin or venial sin, or does the priest tell the Catholic what type of sin the person has committed and what the consequences will be?


The "how" is asking for the process by which the young Catholic will be notified of the sin
Ordinarily, people (children and adults) simply tell the priest in confession what they did. The priest will determine whether the sin was mortal or venial (or perhaps no sin at all), possibly after asking the penitent some questions to get a clearer idea of the penitent's intention and state of mind, or of the circumstances surrounding the sin.  The priest might but need not tell the penitent explicitly whether he considers the sin mortal or venial.