,A practical definition (my own!) for “vanity” would be that which in the big picture , on the whole, does not produce any benefit.
Childhood and Youth, like a tool, once it has been used to perform its intended tasking, it is “put away”.

Through my own consciousness experiencing constitutional emotions, feelings, appetites and desires, I intuit that happiness is valuable and misery is to be avoided, and I know this before I am able to speak .
So I now know the concept of Right and Wrong, and of Good and Evil!
This is developed IN ME (God has shown it to me) and I am not able to deny it.
However, beyond this, being a servant to my constitutional propensities is relentlessly destructive, and I need to “become a man” as soon as possible!

So …

Childhood is absolutely needful, for I LEARN to be a “VALUABLE SEEKER” during childhood. Only when something appears TO ME to be valuable will I CHOOSE it! CONTRAST teaches me to value one thing over another, and I spend much time assigning value to things.

So … why is it that when the MOST VALUABLE BEING (JESUS CHRIST) in any universe is revealed to me, I refuse Him and promote my own selfish value?

After all, having violated God’s Law and incurring the penalty (HELL) instills misery, and ONLY the Atonement by FAITH of the shed blood of JESUS CHRIST is able to cleanse my conscience and remove the guilt and shame!