The suicide talk is a similar. We need to let kids know that there are times in life when things are especially difficult. This doesn't mean they'll feel suicidal at some point, but you do want to introduce the idea that IF THEY DO have that thought, they need to come to you.
I recommend that you have the initial conversation in middle school, and earlier if you see signs that your child is struggling with depression or feels ostracized due to bullying or other factors. If you're not sure whether it's too early to bring up the subject, talk with your child's doctor about it.
And, remember, life letters aren't only for teens!