I disagree. I think people become strong like plants become strong -- by starting with deep roots. I think that childhood is a time to grow deep roots. Later you can challenge the seedling with wind, drought, and deluge and it will probably do just fine because it will have the firm grounding necessary to weather the hardship.
Much of what gets labeled "character building" is actually emotional trauma that kids, as thankfully resilient as they are, gradually get over. But I don't think that "getting over" happens without a cost -- often the result is a subtle emotional guardedness, a "hardening", a wariness, a fear of being hurt, a reticence to commit, a tendency to look to others for approval, to try to please their peers or avoid attracting attention. Growing up tough enough that you can shrug off others' hurts sounds like strength of character, but I think it's hardness of character. True strength of character comes of knowing deep down who you are, and knowing that you are supported and loved for who you are, so that hurts don't damage your sense of your true self.
Many of us view the traumas we went through as kids is something ultimately worthwhile because they made us stronger. I confess I used to believe that myself. But now I think we like to believe that because the alternate interpretation is to awful to contemplate -- that the traumas we went through were entirely useless and unnecessary and wrong, that they should never have happened, that we would have been emotionally healthier people if we'd been protected from those things. That's saying "I went through all that for nothing?!!!" It's not a very welcome conclusion, but I think it's close to the truth.
Two of my kids are now teens. As youngsters they were very much protected from hurtful comments, abusive friendships, exclusionary social tactics, bullying, anxiety-provoking social situations, aggression, power-plays and such. They've grown into very strong people with strong senses of who they are. They shrug off the hurtful language and behaviour of others with no difficulty. They are navigating with the social minefield of teen relationships with confidence and matter-of-fact good sense. Strong roots, I think.
Miranda