I had fairly intense speech therapy for about two years, starting when I was seven. It is worth nipping in the bud early on, as it could lead to teasing at secondary school. I had two therapists, one was too strict and made it a real chore, the other was far more pleasant, and I was allowed to go play afterwards (important to have something to look forward to as otherwise it can seem like your unfairly being given extra lessons).
One of my most vivid early memories was when I could say the word 'treasure' properly for the first time, at which point the therapist said, that's it your done.