A Child at every age has appropriate behaviour for that age.
When a Child of 5 years plays all the time, gets their clothes dirty and fights with their siblings they are acting their age.
When the same person at 16 years plays all the time, gets their clothes dirty and fights with their siblings they are acting Childish.
When I was a child, I spoke as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things. - 1Co 13:11
On the surface this verse seems innocuous. Paul appears to be stating something about himself that is true of most people
So, why did he say this?
The context of this verse is his exhortation on Love.
The Love of God causes Him to be the greatest giver. His focus is not on Himself but on us.
Christ Jesus manifested the selfless Love of God.
God gives us Himself in the HolySpirit.
God is perfect Love, Mercy and Grace.
I am a Child of God and I trust you are too.
I am also a child of my mother who, as I write this, is 93 years old and I am 68 years old.
Though I am a child of my mother, I also am an adult, a father, a grandfather, a husband and a pastor.
So, though I am still a child of my mother I behave as a mature adult (for the most part).
As I think of myself as a Child of God I aught also to think of myself as an adult of God.
This is the point Paul was making.
We are to grow up Spiritually and pass from immaturity to maturity.
And be not conformed to this world: but be you transformed by the renewing of your mind, that you may prove what is that good, and acceptable, and perfect, will of God. - Rom 12:2
When we first come to Christ and are Saved our Mind is conformed to the world.
It remains conformed to the world until we expose it to the Word of God, praying in the Spirit and decide to be Transformed .
A human child has to decide to mature behaviourally before it actually happens. They have to put away Childish things.
Likewise, we as children of God must decide to mature before it actually happens.
Let this mind be in you, which was also in Christ Jesus:
Who, being in the form of God, thought it not robbery to be equal with God: - Phi 2:5,6
We must let the Mind of Christ be in us.
I beseech you therefore, brothers, by the mercies of God, that you present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. - Rom 12:1
We must present our bodies as living sacrifices.
'Deciding', 'Letting' and 'presenting' are acts of our will. They are acts which indicate maturity and lead to greater maturity.
To not 'decide', 'let' and 'present' is to be Childish.
Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God,
Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
And this will we do, if God permit. - Heb 6:1-3
Maturity requires changing what you think about. It requires moving from basic knowledge to higher levels of knowledge.
But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen. - 2Pe 3:18
Growing in the Spirit requires exercising the Spirit through immersion in the Word of God and praying in the Spirit.
When an adult person is accused of being Childish they are typically offended.
Don't be offended. Put away Childish things and move on into maturity.