Until 1929...girls of 12 and boys of 14 could be legally married.
Like most writers I do a lot of research to make my fiction as authentic as possible. I came across this interesting piece of information. Some thing, I might add, I was unaware about. Let me quote Jane Robinson from her book 'In The Family Way.'
"Until 1929, astonishingly, girls had only to be twelve to marry legally, and boys fourteen. So ostensibly, the child of a married couple in their early teens, or of a fourteen-year old boy and a wife twice his age, was perfectly acceptable, while the offspring of an independent, educated woman in her late twenties and living with the man who was to become her partner for life was a disgrace."
Fortunately, the Marriage Restraint Act of 1929 brought an end to marriages at those ages However, it was replaced by 21 as the age at which anyone could marry without the consent of parents. This of course created problems for anyone in Sixties who was under the legal age and could not marry. (Caitlin and Adam in the novel).