The catholic church teaches that Mary never sinned and was saved at birth. The catechism states (#493 and #494)

By the grace of God Mary remained free of every personal sin her whole life long.

Espousing the divine will for salvation whole-heartedly, without a single sin to restrain her, she gave herself entirely to the person and to the work of her Son.

I have yet to read in the Scriptures where this is documented. A doctrine as important as a sinless life, outside of Jesus Christ, is an important topic! It would have been recorded somewhere. Yet, it is not mentioned once.

The Bible, however, is not silent on just whohas and has not sinned.

~2 Corinthians 5:21 For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him.

This is the only documentation in Scriptures of someone being without sin. Yes, Adam was created without sin but his choice led him to sin.

So who did sin? Who was included?

~Ecclesiastes 7:20For there is not a just man upon earth, that doeth good, and sinneth not.

~Romans 3:10 As it is written, There is none righteous, no, not one:

~Romans 3:12 They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one.

~Romans 3:23 For all have sinned, and come short of the glory of God;

~Galatians 3:22 But the scripture hath concluded all under sin.

~Romans 5:12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:

All means all … not everyone except one.

Mary, herself, recognized that she needed a Saviour. Scripture recorded in her own words:

~Luke 1:47 And my spirit hath rejoiced in God my Saviour.

You don’t need a Saviour if you are sinless.

There is no denying that Mary was blessed to have been chosen by God to carry His Son. We do not deny that. However, the whole point was to show mankind that God Almighty would taken on human form to become the ultimate sacrifice for His creation – the Sinless One dying for sinful man. The analogy is incredible – our Sinless God taking on a sinful body so that He could conquer sin and spiritual death – no man could accomplish this task – that is why He had to do it.

Mary was not pre-existing. She was not God and, therefore, could not be sinless and could not leave Heaven to accomplish this task. She was not preexisting, she was born of human flesh and she lived a sinful life. She was a moral Jewish woman, living her life by the law, who still needed salvation. She offered sacrifices for her sin just like everyone else (Luke 2:21-24). She did this to fulfill the law in Leviticus:

~Leviticus 5:7 And if he be not able to bring a lamb, then he shall bring for his trespass, which he hath committed, two turtledoves, or two young pigeons, unto the LORD; one for a sin offering, and the other for a burnt offering.

When one searches the Scriptures for truth, one finds the truth.