I understand what you're asking. It seems that you think that there might be some type of capacity to long term memory. Scientists do not know if this true or not. The answer also depends on the age of the adult. It is clear that senior citizens start showing some memory deficits as a result of age. Cognitive neuroscientists have evidence that indicates that this memory deficit is not necessarily from less brain activity but that the older individuals cannot inhibit activity from non-memory brain areas.

To answer your question, young kids are probably worse than adults. Teenagers and adults are probably as accurate. Older adults will show memory impairments.