A: I was once talking to a friend about this very concept. I came to the following conclusion: yes, we have to go through pain in our lives - because they erase our aveiros and make us into better people. However, at what point in a person's life should they begin to experience pain? And at what degree of discomfort should the painful experience be? Is 9 years old a good time to start? Maybe wait til a person is a little older...until they're 12 years old. But no. That's not how it works. A baby cries to show that s/he is uncomfortable. Hashem knows that in order for us to be able to tolerate and live through pain, He has to start us young - from the very beginning of our lives. Then, when things get harder, more painful, we are able to go through it and it's not like it's the first time we were ever exposed to something that hurts.
To a baby it might be hunger. To a toddler it might be a bump or a bruise. To a child it may be a bigger cut or scrape. A teenager or an adult that gets sick with a sore throat is in pain. Someone breaks their leg, it hurts. That's physical pain.
Then there's emotional pain. To a baby it can be when his mother leaves him with a babysitter - he feels the pain of separation. To a toddler it hurts when someone takes their toys away or doesn't share with them. A first grader might cry when she realizes she left her lunch at home - she feels forgotten. And as a person gets older, there are bigger things that hurt.
Remember that to each person, their painful experience is HUGE. It is their whole life! Whether it's the girl who left her snack at home or the student who didn't do well on her test - it feels just as major to her as the one who failed her road test and the girl who was rejected from the high school/seminary of her choice. It hurts them just as strongly as the potential employee who was told "no" by his prospective boss and the girl who got a "no" from a guy she was dating.
When it comes to feeling with other peoples pain, keep this in mind - don't belittle the pain other people are going through because to them this is their world. This is their life. The pain you are going through may seem greater and it hurts you so strongly but that doesn't take away from the fact that someone else is in pain. So feel with them, listen to them and be there for them - it will help them feel so much better!