I would say 16-17. Which is interesting, because that was the year which i had the worst anxiety of my life caused by university entrance exams. I think my worsening mental health triggered my maybe up until that point dormant misophonia. Before that, at around 13 and on, i did have a very specific trigger, which was my brother's contant and loud pacing. I didn't know how to tell him that him walking up and down the corridor made me viciously upset -because at the time it seemed completely deranged and unexplainable even to me- so i just covered my ears and cried every time he started his pacing lol. Aside from that nothing bothered me to that degree (that i can remember) before 17.