More like lisping, or Elmer Fuddism? (th for s, w for r) I know that if a kid is having trouble with these, in early school years, they recommend a speech therapist; I assume that there's a reasonable success rate. Maybe it isn't so much that initially they can't, but just haven't figured out how? And then, if it goes long enough, the time "window" closes?

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It is hard to convey five-dimensional ideas in a language evolved to scream defiance at the monkeys in the next tree. -- The Science of Discworld III: Darwin's Watch, by Terry Pratchett, Ian Stewart & Jack Cohen