You had plenty money 1922

I think it's a cultural clash thing, to some extent, with differing ages of consent/differing values at different times creating a Does Not Compute. So when a UK reader reads a Merlin story, they are likely to be thinking age of consent = 16, even though it's set in Fantasyland and Fantasytime, but a US reader may be thinking 18. In both cases Merlin, if passing Mordred off as his own biological son, obviously would be assumed to have been underage for his conception and birth, but not as underage for UK readers. (I once came across an argument that fic written for historical fandoms/books written centuries ago should not attempt to depict marriage ages realistically because that is Not How Things Are Done Now, and people like Jane Austen (seriously) just didn't know any better when she has girls marrying in their teens.)