Yes, if the child is old enough to understand. One could also say "respect your elders".
Student or Learner
You need to speak politely to older.
When a kid is speaking rudely to older, how we advise him. Can we use speak politely?
You can't call a person an older. It's an "older person", or "older people" in general.
Take note that older and elder are very different. With your question, you can advise the younger person to: "Speak politely to people who are older than you" or you could simply say: "Stop being rude. Be more polite." I hope that helped