Society has only recently decided that the line between adult and youth philanderings be drawn so harshly. My grandmother, a properly raised Methodist girl from Indiana, married my 26 year old grandpa when she was 14. It wasn't unusual and certainly wasn't illegal.
Understood, hence my earlier reference to my maternal grandmother.
However it can be said that fourteen or fifteen in those times was a lot different from the same numerical age today.
I guarantee you my Mamma-Mia was a lot more mature and adult at fifteen than even I was at nineteen.
Also, most girls in that time period were done with school by age twelve or fourteen, seeing as how it was considered a wasteful extravagance to educate women beyond the basic "three R's" and home economics, sewing and cooking.