It's so weird to me. Not only does the author know the dangers of teenage pregnancy, but they also wish to inform the audience about it. Yet they still have Mitsuo, who should know better, impregnate Tsurihime. Unless there's some plan further down the line to actually criticize his behaviour, I don't get why they're treating his behaviour so lightly. The way these facts are brought forward you'd think those are just the cards that life dealt her, as if this was some unavoidable condition she's been carrying since birth.
She's anywhere from 15-18 when she gives birth and it was two years ago when we first met her so she could have been anywhere from 13-16. However, knowing that would mean the mc was around her age (16-18) at that time, it's safe to assume that she was 14 or under.