How old should I be to use 5kg dumbbells?

Lifting weights. We often advise that kids should not be lifting weights before puberty as it puts additional strain on growing tendons and bones and can affect height and cause injury. That being said, you should only be using a weight that you can hold or move steadily to avoid injury. If you are shaking too much trying to carry 5 pounds or 50 pounds, it's too heavy.