This isn't research and purely anecdotal. But as someone whose grown up with the internet I thought I should share.

I think children can be introduced to the internet at an early age but it's would be extremely unwise to do so without content filters in today's day and age.

Growing up I had unrestricted access to the internet when I was around 4 (this would be circa 2000) and the internet was a much nicer place back then. 

My interests at the start primarily were playing games online, collecting photos of random bacteria and microorganisms, collecting pictures of space, and collecting surreal 3D art in a giant folder (I really need to find those folders).  

The most dangerous vector here was games. I had once in a while encountered questionable ads/content on the games sites I would visit but as a kid my primary interest was the games so luckily that didn't end too badly.

Keep in mind this was 2000. Today its 2023, there are armies of marketers, data scientists, and software engineers being paid 6 figures+ to make those ads as difficult to ignore as possible and very quickly a kid can click a few ads and end up exposed to adult content, end up on a forum with adult topics or generally toxic topics being discussed etc.... 

To recreate the 2000s era internet today would be to offer the same internet but heavily restricting adult content, restricting most social media platforms, and even restricting particular forums like 4chan or certain parts of reddit etc...  

I would even probably just block most game sites (I am sure they are much more addicting today than they used to be)  and require the kid to go through an approval process for game sites to be unblocked. Something like Runescape (while certainly addicting!) is probably not going to lead a dark rabbit hole. OTOH if you encounter a sketchier free games site which could have adult ads, then you might want to wait on allowing access to that until your kid is old enough to know what the negative consequences of that could entail.

Staying along sites like wikipedia, stackexchange, the better parts of reddit, and whatever on google passes some restrictive content filters would be safe. And you probably can in that case offer internet access even as young as 4-6? 

Just keep them away from addicting games, social media, toxic forums, adult content, and violent content. 

The "academic" part of the internet is probably just as fine today as it was 20 years ago when it was almost the only thing in town.

In some sense, maturity might be tracked by how few content filters the kid has in their access to the internet. Starting off with very restrictive and gradually lightening up as they mature. Where day 1 you are probably only allowed to access .edu sites and then the filters gradually lighten up from there.