When do kids start or get the hang of using "an" and "the" (thee) when speaking?

I fear the answer is increasingly "never" based on some linguistics videos I've seen on the topic, but at around what age do or did kids usually get the hang of saying "an elephant" or "thee eggs" rather than "a elephant" or "thuh eggs". Does it get better when they start to write more or is it suffering in writing as well? I need to know whether to work on letting it go more or if I should keep correcting.