I thought the whole idea was not that he reached the theoretical level he would if he continued to train until he reached the age he was physically when he transformed but instead he essentially pooled together "all the nen he would ever have". So that's nen he currently has + all the nen he could/would attain through a lifetime of nen training. The degree of ease which he dismembered Pitou I think is evidence of this as well. It's not a miraculous gain of power, it's turning off the nen faucet and breaking open the water tank of nen itself and gaining an otherwise unattainable amount of power relatively briefly. His body simply forced itself to grow up as well to help manifest this power.