I knew a girl in elementary school that would always say she was engaged, and that her fiance went to a different school. Needless to say, nobody believed either claim.
Until high school, when it turned out to be true.
There was no telling if it was originally just kids playing pretend and turned serious, or if they were serious all along, but they stayed together all through school, and got married not long after graduation. Still together, last I heard.
I want to say she first mentioned it in the sixth grade, but it could have been fifth. I know it was sixth when she would show off her ring, which was a silver band with a tiny little stone in it. Again, nobody really believed her, and there's no way to know if that really was an engagement ring, or what.
So I believe it realistic to say she was engaged at 12, or was at least claiming to be.