Absolutely no clue! I know when I was a very young kid, I never stuttered, but then by 8-9 years old I had an horrifically bad stutter (and I don't remember anything traumatic happening, or medical issues etc). That progressed into my teenage years where it was still really bad, but then calmed down once I hit 19/20, and then has since varied. I only have one Uncle on my mum's side who stuttered when he was younger, and he's now 60 and lost it altogether.... It sucks D: