What does a young person need to know about being at-risk?

Genetics? If you've brought a child into the world knowing that he/she is at risk for, say, huntington's -- were the other questions from you? -- then you have a grave responsibility. You must be truthful, you must not cover over the terrible ordeal that may lie ahead, and you must provide age-appropriate information before the child is a teen and begins making life plans. Get help from the gene counsellor.
At Risk for? It depends on what you are at risk for. For example if you are at risk for being obese you may need diet & exercise.