I kept a diary from age 13 (teenage angst) to 24 (married).
At age 50 I started again and write every night. It's useful to check things but I rarely look back over the piles of blue notebooks.
I don't write anything very intimate and wouldn't much care if anyone read them but they'd soon give up, my life is pretty humdrum.
My mother burnt her many years of diaries, along with her wartime letters from my father but left a couple which I read long after her death. There are fascinating details of her nurse training, her meeting and falling in love with my father, her time in a sanatorium with TB, the start of WW2 , her marriage, life in wartime. Her diaries end during her first pregnancy with twins.
It was funny to see that my mother marked the start of each period with a X, just as I did 25 years later.