There are no historical documents that note how old Mary was when she gave birth to Jesus, so this is a complete fabrication, as it is based on speculation.

Not that I advocate for the UN banning loli doujins, cause that's dumb.

I simply looked it up on Wikipedia.

"Some apocryphal accounts state that at the time of her betrothal to Joseph, Mary was 12–14 years old. According to ancient Jewish custom, Mary could have been betrothed at about 12. Hyppolitus of Thebes says that Mary lived for 11 years after the death of her son Jesus, dying in 41 AD."

From any accounts we have, she must have been in her mid teens at most. I know, it doesn't paint a nice picture imagining that, but historically…