For me, it was around the age of 13 years old. When my maternal grandmother died, my mom went to pieces. In some respects, the dynamic reversed, where I was the mom and she was the child. Albeit, a psychotically deranged child.

My clearest memory is of her playing the song "Everybody Hurts" by REM over and over and over again. Anyone who dares to play this song in my presence these days runs the risk of having their stereo broken.
It got worse when I was 15. Mom's eyes started going on her (diabetic retinopathy) and she had two or three eye surgeries to try and correct the problem. So, there I am, 15 years old - an age where my biggest worries should be schoolwork, a few household chores, maybe a Saturday job and getting to spend time with friends, right?
No. I was cooking, cleaning, doing the laundry, mowing the lawn, running the household, doing the shopping and paying the bills at the local post office. Every morning before school and before bedtime every night, I bathed & rinsed Mom's eyes and changed the dressings. If memory serves me right, there were a couple of days where I had to miss school to go to eye specialist appointments with her. I effectively had no life of my own and there was next to fuck-all help from the relatives at the time. Thank the Gods for having music and books as an escape route, otherwise matricide would have been a near certainty.
Somehow, some way, I also managed to come top of three classes (Art, Woodwork and Programming Principles) and get an honour award for English at high school that year.
In hindsight, I effectively had no "adolescence", per se. It was a straight shot from childhood to adulthood.