The magical number 7
For the normal human listener, holding on to the sentence in perceptual short-term memory is no problem. We are wired for this (neurologically speaking) and improve upon the network as we mature.
We can easily hold the sentences long enough to abstract their meaning and then, dismiss them and go on to the next one.
The magic number for memory length in perceptual short-term memory is 7 chunks of information. For the five year old child, it would be closer to 5 chunks.
We can typically pack more into a message, however, by using strategies to increase the size of each chunk.
Here are some interesting notes on digit span on the Web in the notes below.
NOTES: Get more information on Digit Span
NOTES: Still more information on Digit Span
NOTES: Here is more than you could ever want to know about Digit Span.