Ugh! I apologise - she would have been 26 in 1777! No underage to worry about. Sorry, I'm getting confused with potential John Kneen husbands. Somewhere in my family research someone linked Mary Joughin with a John Kneen born in 1759. I figured this is likely to be wrong, as Mary born 1751 made sense but her marrying a husband 8 years younger probably didn't, which is why I'm trying to find the right John Kneen.
So confusing...

Malea