Both my nephews seem to have speech issues. The 8 year old has major trouble with Rs and is often not understandable. The 3 year old has problems in general so much so that my 3 and 4 year olds don't want to play with him because they can't understand him (we are working with our kids, don't worry about that).
I have spoken to my sister about it and she is basically in denial. She sometimes makes the 8 year old repeat himself, but even then he doesn't really have the skills to fix the word properly.
When is too young to diagnose and help with a speech impediment and when is it too late to truly fix the problem?
At a stammering conference a participant commented that her parents remember she stammered/stuttered on the first word she pronounced. Therefore diagnosis can be made immediately. Regarding addressing, the approach has to be age related, particularly to avoid the child to become too self-conscious of the issue, but a good speech therapist can address it asap after the diagnosis with appropriate methods.