And the op does not make that claim. However, we know that the bible does not state Mary's age. We also know from actual history, that Jewish girls era were setup in arranged marriages around 12 years of age and that marriage was held by no later than her 16th birthday.
The Bible says she was known to be a virgin prior to "coming together" with Joseph. So, we know she wasn't married before. There's no notation about her being older, so we can go with she was a normal girl in the normal world- she was pledged to marry as early as 12.
Because she's a normal girl, she's going to have a normal gestation cycle (and the Bible doesn't mention anything about sped up cycle, so we're going with it). 9 months. Being a normal girl of her era, she gave birth around 15 to 16 years of age and Gabriel gave her the news when she was 14.
No matter how you wanna slice that, Mary was a child.