How old do kids have to be before outlets don't need to be covered?

4-5 years: I find most 4-5 year olds have enough conversational skills to understand danger & the virtue of not messing with something that could hurt them. Your explanation of the "hurt" need only be brief & mention a trip to the hospital for "burns" and tests. Most will accept the caution if they understand burns. Some 3 year olds might but i don't feel comfortable saying they would.

4 or so...: However, make sure you have a talk with your kids about outlet safety, as many older kids will often be found trying to plug stuff in themselves, including things that shouldn't be like phones, etc.