The Catholic Encyclopedia says:
[Based on] the age at which Hebrew maidens became marriageable, it is possible that Mary gave birth to her Son when she was about thirteen or fourteen years of age. No historical document tells us how old she actually was at the time of the Nativity.
How may we reconcile the Blessed Virgin Mary's possibly young age with the evident maturity of her speech in scripture? For example, consider The Canticle of Mary, Luke 1:46:
And Mary said:
“My soul proclaims the greatness of the Lord;
my spirit rejoices in God my savior.
For he has looked upon his handmaid’s lowliness;
behold, from now on will all ages call me blessed.
The Mighty One has done great things for me,
and holy is his name.
His mercy is from age to age
to those who fear him.
He has shown might with his arm,
dispersed the arrogant of mind and heart.
He has thrown down the rulers from their thrones
but lifted up the lowly.
The hungry he has filled with good things;
the rich he has sent away empty.
He has helped Israel his servant,
remembering his mercy,
according to his promise to our fathers,
to Abraham and to his descendants forever.”