I think it’s easier to tell when someone is a child, like still in elementary or even early middle school. Like the writing style and sometimes the way they interact will give it away. But sometimes they’ll still say they’re an adult and there’s not much else you can do. When an older kid or teen is a decent writer (or when someone is an adult learning the language or just doesn’t have the writing skills yet), it gets harder to tell. 

I think 18-21 and up to 25 is a lot harder to tell, but maybe someone a couple of decades older might be able to sus out the vibe better!