The New Testament writers show great concern for the spiritual growth of their readers.  Paul, for example, writes to the Corinthians, “But I, brothers, could not address you as spiritual people, but as people of flesh, as infants in Christ. I fed you with milk, not solid food, for you were not ready for it. And even now you are not ready” (1 Cor. 3:1–2).

The writer to the Hebrews similarly: “For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food” (Heb. 5:12).

All believers, of all ages, should grow and mature in the faith.  John recognises that some of his readers are ‘children’, others ‘young men’ and still others, ‘fathers’ in this regard (1 Jn 2:12-14).

This concern crops up repeatedly:

“Until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, so that we may no longer be children, tossed to and fro by the waves” (Eph. 4:13–14).

“And it is my prayer that your love may abound more and more, with knowledge and all discernment, so that you may approve what is excellent, and so be pure and blameless for the day of Christ, filled with the fruit of righteousness that comes through Jesus Christ, to the glory and praise of God” (Phil. 1:9–11).

“I know that I will remain and continue with you all, for your progress and joy in the faith” (Phil. 1:25).

“Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own.… I press on toward the goal for the prize of the upward call of God in Christ Jesus. Let those of us who are mature think this way” (Phil. 3:12, 15).

“Therefore, as you received Christ Jesus the Lord, so walk in him, rooted and built up in him and established in the faith, just as you were taught, abounding in thanksgiving” (Col. 2:6–7).

“Train yourself for godliness” (1 Tim. 4:7).

“Let us leave the elementary doctrine of Christ and go on to maturity” (Heb. 6:1).

“Like newborn infants, long for the pure spiritual milk, that by it you may grow up into salvation” (1 Peter 2:2).

“Make every effort to supplement your faith with virtue, and virtue with knowledge” (2 Peter 1:5).

“But grow in grace and in the knowledge of our Lord and Savior Jesus Christ” (2 Peter 3:18).

See J.I. Packer & Gary Parrett, Grounded in the Gospel Building Believers the Old-Fashioned Way, ch. 7.