I’m never sure, cause mine started around like 8/9 and was it’s worst around 10, which is when I had to get therapy because I could hardly speak. According to online sources, it usually starts around 2-6 if it’s a developmental thing, and I could actually speak very well when I was little. I guess that’s why speech disorders should have their own category, because so many things can cause them. Causes can be developmental, anxiety-based, injury-based (from strokes, etc), trauma, and so on. And for me, the selective mutism part makes it fall under an anxiety disorder (which is also not an option). Honestly, I’ll probably just leave it blank.