I had one in California. We were both 12 and our birthdays were only a day or two apart. We wrote to each other for about 7 or 8 years until we both got seriously involved with boyfriends!
In the early letters, the subject matter was mainly about fashion and pop music but it changed as we grew older. Boyfriends, sex, the civil rights movement and the Vietnam war dominated our correspondence. Her best friend wrote to my best friend so some interesting exchanges went on!
Sadly, her friend, who married at 17 was widowed some months later when her husband was killed in Vietnam.
I kept the letters for a long time but eventually got round to throwing them away. I wish I'd kept them now.