I believe that your average rate of personal growth is highest at birth, and decreases over time until death. To put it another way, the increase in your mental and emotional growth between 2 and 3 years old is wildly greater than between 80 and 81 years old.
2-3 year olds also cry a lot more than 80-81 year olds. Even though the reasons they cry might be silly, the pain to the 3 year old is real. They’re experiencing the same level of hurt as an adult does when they cry.
It might be a coincidence, but I think the increased amount of pain children experience is related to their personal growth.  I don’t have any supporting details for this theory besides: 1) pain and personal growth rate seem to drop off at the same rate over a lifetime, and 2) the biggest improvements I’ve made in life seem to be after a period of pain.
The remainder of this essay is personal, so feel free to skip it. It is uncomfortably personal for me.
A large period of pain for me was in the beginning of 2014. In a period of a few months, my parents divorced, my relationship ended due to my partner’s infidelity, my closest friend at the time was unavailable for emotional support, and my business was shut down (and as a result, my relationship with investors ended). It was very tough, but I truly believe I was stronger after it. Going through pain gives you a fearlessness, like “It doesn’t matter what else happens because it won’t be worse than that.”
I’m going through a similarly tough period now. In the past month, I ended my relationship due to irreconcilable differences, I let my personal spending balloon unsustainably, I had my personal worst interview performance, and I’m now living alone for the first time, which does make it a little harder. Yesterday, I was eliminated on the first day of a televised business competition, which added to the frustration. 
My hope is that this will result in another period of growth for me.
As a disclaimer, I of course want this theory to be correct because of what I’m experiencing now. I’m currently biased.
 To clarify, pain != growth. Kids that grow up in abusive households have a tougher life, even though they’ve experienced more pain. It’s about the pain that results from growth.
 I’m happy to report that the loss on the competition only slightly affected my mood. I know it was for two reasons. 1) I have enough experience in business to know that the loss is not a reflection of my business acumen, but rather what makes better reality TV. 2) It is not as serious as a loss of a relationship, so that helps me realize its insignificance.