I would tell them they will be disappointed, that they will someday see the world in its entirety and be appalled that humanity wastes so much energy on conflict and greed, so much time fighting ignorance and apathy. You get to a certain age and level of awareness and it’s “Oh, goodness, what a mess.”
I would tell them they will experience disappointment with the people closest to them, their family and friends, that their behavior will not always be what it can or should be. And that they will also sometimes feel frustrated and disappointed with themselves, that they will not consistently be the person they aspire to be.
But also…I would tell them that, imperfect as we are, that we can choose our company carefully and band together with others who strive toward the good. That not everyone is a lying manipulator, that there are many who are earnest and sincere. Not perfect, but thoughtful, positive, interesting. Not naive, but knowledgeable, striving, bold.
That we can be those people, we can light the way for others. That we can leave the world maybe just a little or a lot better than the condition we found it. That our task is to promote kindness and encourage persistence, to keep trying, to improve with patience and practice.
I would tell them: you will feel disappointment. But set that aside. There are pleasures to enjoy and goals to work for. That’s what living is.