I didn't know what I was experiencing was "parts" because I neve knew anything different. It was just always my normal and I never thought to question it ,I never talked about it or even thought twice about it cause how was I to know it wasn't how everyone else on the planet experienced life or anything out of the ordinary?
It wasn't until I had been in therapy quite awhile that I was informed that parts of myself had been presenting themselves from the very beginning, even through email before my first session. So I guess that's my answer to when. And my age was in my 40's. But if you're meaning when I realized the actual parts myself, that took quite awhile after being informed by my therapist that I had them.