I'm not familiar with this phrase and therefore don't associate it with elementary school children, so if I heard someone say it, it wouldn't strike me as being childish. It actually sounds to me like something an adult would say to a child to teach them to not be nosy.
Like some others have said, until you mentioned the threat aspect, I didn't get that intent. I just thought it meant "good people live longer."
If it was said in a lighthearted manner to someone you're close to, I wouldn't consider it rude. And, if you said it to your great-aunt Mary who keeps hounding you about when are you going to have kids or lose weight, etc., I don't think she'd think you were threatening to kill her if she didn't shut up.
When I was a kid, we used to say "mind your own beeswax." I just looked it up and saw that this phrase started in the 1930s, which surprised me because I thought we were being very original in the 80s.