Luke, the author of the gospel that bears his name and also of Acts, was an exceptional historian of the classical mold. Roman and Greek history was a little different than what we expect from the field. Wikipedia suggests a typical approach:

In Roman historiography commentarii is simply a raw account of events often not intended for publication. It was not considered traditional “history” because it lacked the necessary speeches and literary flourishes. Commentarii was usually turned into “history” later on.

A Roman reader would expect the author of a history to present a polished account rather than a word-for-word stenography. Given the expense of the two volumes Theophilus commissioned, he would have demanded the high literary quality that Luke did, in fact, deliver. Therefore, Peter's sermon in Acts 2 was likely "punched up" a bit.¹

We know that Luke used a number of sources. Luke's version of Jesus' genealogy and the details of the Nativity which seem known only to Mary² suggest that Mary herself was one of Luke's sources. If so, we may conclude that Mary relayed the Magnificat to Luke and he wrote it down. In all likelihood, she would have recited it 30 or more years after the event in Aramaic. So we aren't reading a teenage girl's poem, but a Greek translation of psalm from the heart of a widow who has outlived her oldest son.

This is not to say that Mary could not or did not compose the canticle that we read (modulo translation, of course) nor that it was remembered from her youth. Rather, we ought not be surprised by the literary quality of soliloquies in the Bible and especially in Luke's works. Ancient writers were far more concerned about the essence of history than its substance. Without a doubt, Luke portrays the pregnant Mary as wise beyond her years. The song he provided her with reflects that reality.

Footnote:

1. Or maybe not since he was inspired by the Holy Spirit at the time. But the point is, Luke didn't have a tape recording to transcribe and he likely did not have the precise words of the sermon. Rather he gathered one or more accounts of what was said, quoted some relevant Scripture, and "reconstructed" the speech.

2. Particularly:

   But Mary treasured up all these things, pondering them in her heart.—Luke 2:19 (ESV)

Luke, the author of the gospel that bears his name and also of Acts, was an exceptional historian of the classical mold. Roman and Greek history was a little different than what we expect from the field. Wikipedia suggests a typical approach:

In Roman historiography commentarii is simply a raw account of events often not intended for publication. It was not considered traditional “history” because it lacked the necessary speeches and literary flourishes. Commentarii was usually turned into “history” later on.

A Roman reader would expect the author of a history to present a polished account rather than a word-for-word stenography. Given the expense of the two volumes Theophilus commissioned, he would have demanded the high literary quality that Luke did, in fact, deliver. Therefore, Peter's sermon in Acts 2 was likely "punched up" a bit.¹

We know that Luke used a number of sources. Luke's version of Jesus' genealogy and the details of the Nativity which seem known only to Mary² suggest that Mary herself was one of Luke's sources. If so, we may conclude that Mary relayed the Magnificat to Luke and he wrote it down. In all likelihood, she would have recited it 30 or more years after the event in Aramaic. So we aren't reading a teenage girl's poem, but a Greek translation of psalm from the heart of a widow who has outlived her oldest son.

This is not to say that Mary could not or did not compose the canticle that we read (modulo translation, of course) nor that it was remembered from her youth. Rather, we ought not be surprised by the literary quality of soliloquies in the Bible and especially in Luke's works. Ancient writers were far more concerned about the essence of history than its substance. Without a doubt, Luke portrays the pregnant Mary as wise beyond her years. The song he provided her with reflects that reality.

Footnote:

1. Or maybe not since he was inspired by the Holy Spirit at the time. But the point is, Luke didn't have a tape recording to transcribe and he likely did not have the precise words of the sermon. Rather he gathered one or more accounts of what was said, quoted some relevant Scripture, and "reconstructed" the speech.

2. Particularly:

   But Mary treasured up all these things, pondering them in her heart.—Luke 2:19 (ESV)