I've done "instant reefs" before by simply moving live rock from one tank I'm tearing down into a new one I'm setting up. However, I usually have multiple tanks set up at any one time, so I have an "insurance" factor built-in in case something goes wrong, like a nutrient spike or bad algae bloom. So yes, using mature live rock from an existing system can cycle your tank, but there's always a small chance of die off (like sponges from being exposed to air during the transfer). I would NOT call any of these "instant" reefs mature right away.
A mature reef is one that is no longer in danger of going through the normal growing pains or has already experienced and gotten past issues like diatom or algae blooms that is quite common for newly set up reef tanks. Rather than applying the term "mature" to a specific time frame, to me it means the tank has already stabilized, corals are growing well, and there is a growing biodiversity as arthropods, copepods and/or live mysis are now breeding and multiplying in the rockwork and/or sump. And yes, how soon one reaches that stage of tank maturity has a lot to do with the reefer's experience, but it will still take time for everything to settle out and stabilize.
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