In theory, you are correct - but once we reach the age of reason, we need the Sacraments to help us remain free from sin. Without the Sacraments - particularly Confession and Communion, its doubtful at best, if we could remain free from sin for very long at all. As TotusTuus points out, even though we get baptised, we still suffer the effects of Original Sin, other wise know as concupiscience or "the tendancy toward sin".
Our Blessed Mother, being born without Original Sin, was not subject to concupiscience. Our Blessed Mother, lacking the essential ingredient for sinning therefore lacked its effects or its tendecies - rather, Her tendancy was always toward God and growing in His grace.
That is perhaps the most difficult thing for us to imagine because we struggle to remain in the state of grace, but Our Lady constantly grew in the state of grace.
|5-December-2004 -- Vatican Information Service|
CONCEPTIONVATICAN CITY, DEC 5, 2004 (VIS) - Pope John Paul dedicated today's Angelus reflections entirely to the upcoming feast of the Immaculate Conception, as this year marks the 150th anniversary of the proclamation of this dogma by Pope Pius IX.
"We prepare to celebrate with intimate joy," he said, "the solemnity of the Immaculate Conception of the Blessed Virgin Mary which this year takes on special meaning. We will commemorate, in fact, the 150 years since the proclamation of this important Marian doctrine.
"We will remember this anniversary on December 8 with a solemn Eucharistic celebration in St. Peter's Basilica where, in 1854 my venerated predecessor, Blessed Pius IX, proclaimed the dogma of the Immaculate Conception, together with many bishops from every part of the world. Thus we will honor 'Tota pulchra', she whom God chose as the Mother of His only begotten Son."
The Holy Father closed by reminding everyone that in the afternoon, as he does every year, he will go to Rome's famed Piazza di Spagna for the traditional homage to Mary Immaculate. "I invite all of you, dear Romans and pilgrims, to join me in this act of filial veneration to our heavenly Mother."