How old was Mary when she had Jesus? I've heard that she was between 12 and 14 years of age. Is that in the Bible?
To my knowledge, there are no Scriptures that clearly address the matter of Mary’s age. I think that the age range you have suggested is popular, and may even be right, but it really is a matter of speculation – something we’re cautioned about (1 Timothy 1:4; 2:23). Deuteronomy 29:29 should probably be our guide here.
The bottom line is that it is not really important, which is why we were not told in specific terms.