There are some cases when very young children are treated with the Lidcombe Program. However there is always a significant reason for doing so. Some examples are:
a) the child and parent are showing significant and consistent distress and the speech-language pathologist feels the child would respond quickly to treatment,
b) the parent has already treated older children and has indicated they will go ahead and attempt to treat the younger child without guidance from the speech pathologist despite being advised not to or
c) the family is moving overseas and will not be able to get treatment there.
Generally, even in these cases, it is advisable to wait for 6 months post onset, as it is in this time that the chances of natural recovery are reported to be greatest. To answer your third question, there is no direct research to show that very young children take longer to treat with the Lidcombe Program than older children. However our clinical experience suggests it can. Very young children can be more difficult to keep in one place for extended periods making it difficult for parents to conduct structured therapy sessions. The Kingston et al (2003) paper showed no significant difference between treatment time of 3-4 and 5-6 year olds, however it was shown that children who were more than 12 months post onset took slightly less time to reach Stage 2 than those who were within 12 months since onset.
For a thorough discussion on timing of intervention with the Lidcombe Program I recommend you read chapter 4, by Dr Ann Packman, in the book “The Lidcombe Program of Early Stuttering Intervention: A Clinician’s Guide”.