The correct answer is, of course, "Why would I want to grow up?"
That's still the one I use.
Why, indeed...
11 And He Himself gave some to be apostles, some prophets, some evangelists, and some pastors and teachers,
12 for the equipping of the saints for the work of ministry, for the edifying of the body of Christ,
13 till we all come to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ;
14 that we should no longer be children, tossed to and fro and carried about with every wind of doctrine, by the trickery of men, in the cunning craftiness of deceitful plotting,
15 but, speaking the truth in love, may grow up in all things into Him who is the head — Christ —
16 from whom the whole body, joined and knit together by what every joint supplies, according to the effective working by which every part does its share, causes growth of the body for the edifying of itself in love.
— Eph. 4:11-16 NKJV
1 Therefore let us go on and get past the elementary stage in the teachings and doctrine of Christ (the Messiah), advancing steadily toward the completeness and perfection that belong to spiritual maturity.
— Heb. 6:1 AMP
And not forgetting to properly acknowledge this instruction:
15 Truly I tell you, whoever does not receive and accept and welcome the kingdom of God like a little child [does] positively shall not enter it at all.
— Mark 10:15 AMP
For we here recognize that the directive is that we must accept The Kingdom in childLIKE — not childISH — fashion, which ability has no discord with growing up to maturity.
Good point.