To me a mature tank is teeming with life, live rock have sponges on them, a healthy population of copepods, amphipods, etc that can feed demanding fish, stable water parameters, and the tank only need occasional water change, etc. A mature tank take years to develop. It is so stable that you can take off your protein skimmmer and tank still thrives.
This is exactly my thoughts, and statement above. Pods, sponges, dusters, tunicates, coraline algae, micro stars, worms, etc... Many tanks today devoid of this as many tanks established with dead sand and rock. Overtime, when buying corals or introducing new life, they will seed, the tank, but this takes time.