We have sinned and grown old
There's a quote by Chesterton that I just love:
A child kicks his legs rhythmically through excess, not absence, of life. Because children have abounding vitality, because they are in spirit fierce and free, therefore they want things repeated and unchanged. They always say, ‘Do it again’; and the grown-up person does it again until he is nearly dead. For grown-up people are not strong enough to exult in monotony. But perhaps God is strong enough to exult in monotony. It is possible that God says every morning, ‘Do it again’ to the sun; and every evening, ‘Do it again’ to the moon. It may not be automatic necessity that makes all daisies alike; it may be that God makes every daisy separately, but has never got tired of making them.
It continues:
It may be that He has the eternal appetite of infancy; for we have sinned and grown old, and our Father is younger than we.

Defining Maturity
Sometimes people mature because they pursue maturity.  Sometimes people mature because life matures them.  For me, it was a weird mix of both.  People started telling me I had an "old soul" starting somewhere in Jr. High.  I was always "old" for my age, but now age is catching up with me (as it does with us all).

As a kid, I didn't laugh much.  For one thing, I didn't have a whole lot of reason to.  But for another thing, somehow I equated "maturity" with "solemnity."  I distinctly remember being twelve and forming the thought.  (Yeah, I was a weird kid).  In my pursuit of maturity, my immaturity had caused me to construct the wrong notion of maturity.  It's funny how that happens.

Just as it is important to have at least a working framework for defining manhood, womanhood, or what it means to help someone, I believe it is also important to have at least a general idea of what maturity is.  If we're going to strive for something, it's good to know what we're striving for.  Then again, this may or may not have something to do with the fact that I'm a law student who likes words and definitions.

Missing out
There is something my 10-year-old brain missed: that's there's an incredible amount of joy in life, and it's OK to enjoy it!  That's something that [most] kids understand.  They have an innocence about them that can appreciate the small things.  But somehow, we lose that.  We have been sinned against, and we have sinned, and we have grown old, and our Father is younger than we.

Now, if you run with the Chesterton quote too far, you'll probably end up with some funky theology.  However, it gives a great counter-point to what many conceive as mature, adult living.  We become cogs in a machine.  We wake up, brush our teeth, drive to work, clock in, clock out, drive home, veg on the couch, and go to sleep.  We forgot the excitement in life.  We're missing out.

I believe this is one of a myriad of reasons why we must come to Christ with the faith of a child.  Why we must be reborn.  Why Christ gathers the children to himself and fiercely protects them.  Because there is an appetite in infancy that aligns with the Divine Nature.  Because there is maturity in childishness.