I noticed I had anxiety when I had a panic attack in the car. I woke up feeling so anxious I didn't want to leave the house, but I did anyway and in the car I had a panic attack. This put me into a deep depression/anxiety feeling for days (mostly because I was so sad and afraid something was mentally wrong with me) and it got so bad I felt like going to the hospital.
I had a lot of symptoms of it before, like head tremors in large social settings or increased heart rate when I would enter classrooms, stuff like that, but I ignored it and thought it was normal and just a symptom of my self consciousness. It got much worse after I had a traumatic experience and the problems at home combined to give me depressive/anxiety episodes of non stop thinking and emotional feelings.
Now that I look back it seems I have had anxiety since a young age, most likely after a few traumatic experiences.