Many children from age 18 months to 5 years old have occasional initial syllable or word repetitions, or use of filler pauses ("Umm ...") - this is called disfluency, and is usually a normal developmental stage where the mind is thinking of words faster than the mouth can utter them.
True stuttering is more common in boys, if there's a family history of stuttering, with later onset of symptoms, if there are speech/language delays, and if the stuttering is more often present than not, especially if present for 6-12 months or more.
Some characteristics of true stuttering include:
- frequent repetitions of sounds, syllables, or short words
- frequent hesitations and pauses in speech
- absence of smooth speech flow
- tense facial expressions or facial tics
- a fear of talking