Well, I remember things that happened when I was 12 years old and I remember my grand parents quite well even though they died before I was 6 years old.

I imagine that the grand daughter (who may have been 16 as my memory is unclear) had, or at least listened to, conversations with her mother and other relatives.

[I can tell family stories and name names of people who were deal long before I was born, but among Southerners there is a long oral-tradition which is handed down from generation to generation.]

Of course, you are correct in assuming that a 12-year old might not remember well enough to be taken seriously.