Question: What’s some knowledge that you’ve gained over your lifetime that would’ve changed your life if you understood this as a child/ teen?
Answer: I am sorry to answer “negatively”, but I do not think “knowledge” gained in adulthood would have helped us retrospectively in childhood.
We are primarily emotional creatures, our “understanding”, “mind” develops through emotional impressions. And we gain emotional impressions by experience, by acting out, performing certain roles, living through situations.
As they say nobody can truly understand their parents until they themselves become parents. We can argue about certain people and their actions, we can criticise and judge them, but we can never understand them and what actually happened, unless we “stand in their shoes”, see the world through their eyes, go through their conditions.
Thus for true understanding we need maturity, experience, wise people say that “none as wise as the experienced”. So even if we “received” some adult knowledge about something as child it would not have helped us, since first we need to grow up and go through the same, true life experiences in order to know how to use that knowledge.