Mine started a week after my 12th birthday, so, first week of March 1977.
If there was no drama to it, and it wasnt on or near something memorable like a birthday or holiday, i can see being vague as to the exact date. But a 5 year range, that stretches from primary to secondary school, from playground to perfume (as the song says) from possibly a rather early 10 to a kinda of late-ish 16, seems unusually vague!
Having moved a lot, i have answered this question numerable times, usually with the follow up question regarding regularity, severity if symptoms, number of pregnancies and how they concluded. Individually the seperate answers dont mean much, but collectively, they might. And until your other records catch up AND the doctor has reason to read them, they are part of the skeleton of your medical history.
When my dds were approaching menses, I asked my gynae about things that happened to me When I began that I had never seen as symptoms for the beginning, but was reading about as symptoms of ending. I spent the first year with horrible hot flashes. I didnt have a name for them. My mother acted as if I was just being whingey about the summer heat and probably had no idea either. I got in trouble for moving extra fans into my room in the middle of the night, for wasting water taking uneccesary (COLD) showers. For changing clothes too often on a whim and creating more laundry to be done. I was sweaty, not fashion conscious!
Surprisingly, my doctor said she had not experienced that, nor been taught that, nor heard any other reports. She admitted though, she also had never heard of comprehensive research of girls actually going through the phase. Most of what they know is from adult women reporting what they remember.
Hearing that you cant even recall when, makes me wonder what else many women wouldnt remember and report. I cant be the only girl to have had hormonal hot flashes.