Upon reaching the age of reason, a Catholic is obliged to confess his serious sins. The Church urgently advises the faithful to do this at least once a year. At any rate one must go to confession before receiving Holy Communion if one has committed a serious sin.
By “the age of reason,” the Church means the age at which one has arrived at the use of reason and has learned to distinguish between good and bad.
From YOUCAT, page 136-137