The earlier a child is assessed and starts receiving therapy, the better it is for the child's eventual progress. It's really never too soon for a child the parents suspect of mental retardation to be assessed. Even a six-month-old can be assessed, and if intervention begins early, the child usually fares better mentally and physically.
I think child life specialists who think a child should be a certain age before he or she is assessed are doing a special needs child a grave disservice. Some feel a child should be at least three years old, but therapy can start much earlier and it is very helpful, even before the age of 12 months.