Is there any tradition that informs us of how old Mary was when she gave birth to Jesus?

I have heard that the legal age of marriage was quite young in Jewish culture at the time of Christ. I believe young girls at the age of twelve could marry. However, I find it difficult to imagine Mary as young as thirteen years years old around the time of her giving birth, because of the maturity of her words that are found in the gospels. Is there any tradition that would place a more realistic age of Mary? Or is this just completely unknown even when guessing?