The only way to retain information that long in working memory would be to keep thinking about it, such as through continuous rehearsal. Since an 8 month old is not capable of rehearsal or other ways of keeping the stimulus active, after 5 or 10 minutes (really, after 30-60 seconds) the only way they would recognize the stimulus would be if it had been encoded into, and retrieved from, long-term memory.

The key terminology is that "short-term memory" is not like a shorter version of long-term memory.  It's really a misnomer. STM, which is more often referred to as working memory, is better thought of as the current contents of your consciousness, whatever you are thinking about right now.  Just a few seconds after shifting your thoughts to something else, it's gone, and the only way to get it back is to retrieve it from LTM (assuming it was encoded well enough to get into LTM).

The terminology of STM-LTM needs clarification here. For example, a weakness in many studies is based upon time! How long must a memory persist for the memory to be classified as one or the other? If an infant can "retrieve" a word after 10 minutes, can it be "retrieved" after 15, 30, or 60 minutes? Can it be "retrieved" after 24 hours? Perhaps we are discussing a process here instead of terminology such as STM-LTM. It might be best to discuss the phenomenon by relying upon operational definitions; For example, there is no question about the term immediate memory. But after that point, operational terms leave no question about the matter, as in "STM as assessed by a 20 second interval; a 5 minute interval; LTM as inferred from a 15 min interval; LTM as assessed by a 20 minute interval." The operational definitions specify the behavior and avoid drawing the arbitrary line between STM and LTM. Furthermore, the literature which speaks of the development of the medial temporal lobe memory system characterizes "retrieval" as a sophisticated cognitive function that is evident much later than 8 months; it is more likely that your reference to "retrieval" refers to "recognition," since the behavior described would appear to rely upon the medial temporal lobe memory system - which is essentially a recognition memory system.  Len Koziol

Generally, even short delays constitute LTM. As the others said, STM is merely what is still in one's focus of attention. Typically after a minute or two, attention has shifted to something else, so retrieval is from LTM.

Thank you all! Your feedbacks helped a lot! As you said, I don't think that they are strategically rehearsing the words in working memory. During 10-min delay interval, they are playing with toys and not hearing any of target words. 

As others have already clarified, 10 minutes is definitely after STM; although estimates of short-term activation of representations vary in the literature, their range is usually between 2 sec and 30 sec - unless, of course, information is rehearsed during the retention interval, but clearly this is not the case of 8-month-olds.

It should also be considered, however, that 10 minutes are still within the stage of LTM consolidation; this is a process that takes place in about 20 - 60 minutes (and some say also during sleep). I am not a nuropsychologist, but as far as I can understand it has to do with interactions between the hyppocampus and other cortical structures specific for a certain kind of LTM representations. Anyway, some of the memories that after 10 minutes are still in the consolidation stage become consolidated, and some others have a more ephimeral life because they do not become consolidated LTM units. The prediction should follow that, if you test children after (say) 2 hours, some of their non-consolidated memories are already gone; i.e., you should find better performance after 10 min than after 2 hours.

Samuelson is right. After five minutes information is definitely part of LTM. Consolidation is another matter and it is related to awareness of memory processes and memory needs (e.g., that rehearsal is needed). Infants before 1 year of age are very weak in that, although this kind of studies may shed light on awareness as much as about memory.