I'm not young, so it's hard to pick just one as "most valuable", but there's one thing that has been a thread throughout my whole life, beginning in early childhood (and this is one I think many/maybe most children have but may outgrow) and that's being far more mature and strong and capable than other people often would imagine that someone who looks like me might possibly be. It's pretty common for adults to underestimate those traits in children, and it what can make it difficult is that it can be hard to sort out what is the immature behavior of a child or teen, how a kid might be very mature and sensible and strong in all kinds of ways and yet still not know the same number or type of facts about something that someone older would.
Kids grow up, of course, and in adult life there are all kinds of things that others use as a way to judge how mature, strong, capable, etc. etc. someone else is. They're sometimes external, sometimes subtle, but they're things that get the person with x number of them sometimes even an automatic assumption of being more capable, strong, mature, etc. than he really is - and that's without the person even having to do much of anything other than just have the benefits of those "markers" (for lack of a better word).
Anyway, as a small-framed woman with a "small" woman's voice, and an appearance that doesn't lend itself well (without my turning myself completely ridiculous looking) to attempts to look older or smarter or stronger (or whatever); it's pretty much been a thing that I've always been well aware that people are not always what they appear to be on the outside.
It didn't help that I went from being younger looking than I was for a long time, so somewhere along the way I just went from being seen as "less" because of that to often being seen as "less" by some because of age.
Living with that has made me so aware of NEVER seeing others through a "looks lens", and because of that I never shut off the chances of learning so many other lessons about people and life. I tend to err on the side of assuming strength, sense, maturity, intelligence (etc.) in others - which, oddly enough, is one of things I like most about myself. (Of course, doing that hasn't gotten me a lot of being assumed to be anything but naive and/or "too /kind" (which then further feeds the "dumber-than-I-look" thing). I'm not alone in this challenge, and it can be "a pain". I still prefer it to seeing others through a false lens myself.