I won't get into the biology involved, mainly because I'm not the best one to do that. I'll approach the answer from a different angle... RISK.
You quarantined your fish. You chose the most conservative method for managing the risk of disease. Great choice.
Conservative actions are generally a better way to approach managing risks in a tank. Adding bioload as slowly as possible is another risk management strategy. If you have the capacity to add one fish at a time and allow the bacteria that drive the nitrogen cycle to balance naturally, why not do so? If you can add one fish and observe its impact to the system, why not do so? The risks you are trying to manage here is the possibility that somehow, an ammonia spike will occur and stress all the fish in the tank, and the risk that your system might not have the capacity to handle the additional dissolved organic carbon generated by the addition load.
As far as the "age" of a system... The nitrogen cycle is only one of a host of processes that develop over time in a tank. While early on, it is very important, it becomes less so as other processes develop. We can only do so much to speed up those processes. Age is not a good measure of a tank's capability. The relative maturity (or immaturity) of the processes in a tank can really determine how easy it is to maintain and how resistant it is to our mistakes.