In all honesty, I'd let them have free reign, but be extremely watchful and tell them to come to you if they see anything they don't understand.
For real, considering how everywhere and everything has an internet presence today - and how much it'll likely increase in the future - I think it's impossible to keep children away from it by the time they know basic tech literacy (e.g. knowing how to turn on a tablet or smartphone). There's also the scope of it. Even something as easy-to-access as Youtube contains years and years worth of content, and there's a lot of insidious stuff masking itself as kid's entertainment that, unless you're constantly watching over your kud's shoulder, you can't be sure they haven't seen, and you can't block every website that you know to have kid-unsafe content, since there's about a dozen more that you don't know about.
Thankfully, I think more elementary schools are putting in basic computer literacy as their curriculum, and I think once more and more schools and kids start learning about it, they'll probably expamd it to include internet literacy as well - stuff like being aware of spam and viruses, keeping away from strangers, and how to find actually kid-friendly websites. All-in-all, I think as long as you give a child a good talking to and make sure they're learning safe navigation, you'll be fine.