Then He called a child to Him and had him stand among them. “I assure you,” He said, “unless you are converted and become like children, you will never enter the kingdom of heaven. Therefore, whoever humbles himself like this child—this one is the greatest in the kingdom of heaven.
(Matthew 18:2-4 HCSB ©®)
Jesus answered, “I assure you: Unless someone is born of water and the Spirit, he cannot enter the kingdom of God. Whatever is born of the flesh is flesh, and whatever is born of the Spirit is spirit. Do not be amazed that I told you that you must be born again. The wind blows where it pleases, and you hear its sound, but you don’t know where it comes from or where it is going. So it is with everyone born of the Spirit.”
(John 3:5-8 HCSB ©®)
So it is with everyone born of the Spirit: they are going somewhere! You may not know where they are coming from or where they are going, but they are going somewhere. They are not just the child they had to become like before they can enter the Kingdom. Now, they have entered. Now, they need to become mature in the Kingdom, by being changed and by being obedient to the will of the King.
Maturity in the Kingdom is measurable by presence of the fruit of His Spirit, and how quickly it can be measured.