Spiritual growth and maturity do not relate to age.
When we are first born again, we are considered spiritually immature, regardless of our age.
Ephesians 4: 11-15: "So Christ himself gave the apostles, the prophets, the evangelists, the pastors and teachers, to equip his people for works of service, so that the body of Christ may be built up until we all reach unity in the faith and in the knowledge of the Son of God and become mature, attaining to the whole measure of the fullness of Christ. Then we will no longer be infants, tossed back and forth by the waves, and blown here and there by every wind of teaching and by the cunning and craftiness of people in their deceitful scheming. Instead, speaking the truth in love, we will grow to become in every respect the mature body of him who is the head, that is, Christ"
2 Peter 3:18: "But grow in the grace and knowledge of our Lord and Savior Jesus Christ"
1 Peter 2:2: Like newborn babies, crave pure spiritual milk, so that by it you may grow up in your salvation"
1 Peter 2:21: For to this you were called, because Christ also suffered for us, leaving us an example, that you should follow His steps" (grow up spiritually like Jesus)
As Christians, we need regular nourishment of God's word, reading and applying God's word to our lives. We must be taught, rebuked, corrected and trained by God's word. This equips us. Also - we need to walk in the Spirit, allowing the Holy Spirit to guide us in thought word and deed (Romans 6:11-14)
James 1: 23-25: "For if anyone is a hearer of the word and not a doer, he is like a man observing his natural face in a mirror; for he observes himself, goes away, and immediately forgets what kind of man he was. But he who looks into the perfect law of liberty and continues in it, and is not a forgetful hearer but a doer of the work, this one will be blessed in what he does"
The scriptures do show us what we look like Spiritually and shines light on the areas where we need to grow.