I know this is 4 months old but a response seemed necessary on this one; I developed my stutter when I was 13. Before then I was fluently spoken. I think it developed from some trauma and bullying and a lack of overall confidence So I imagine it's perfectly possible to develop stutters even in later life. As an adult I'm much better now but it is still there in some words, very specific sounds (for me it's on the vowels) cause my stutter. So there's a pattern to WHEN I stutter as well.