I think that online exposition should be managed through each age by choosing the appropriate device/online filtering:

1st stage - offline tablet with selected apps.

2nd - laptop with internet content filter in a separate device (e.g. router openwrt with adguard).

3rd - Smartphone with parenting control

But the most important part is education. This is just as using that helping wheels when learning how to ride a bicycle, you need to ensure that your children is ready when you take off the safety, so they don't fall.

I think the age depends on the parent’s parenting style.

Our kids both got devices very young. That said, the devices weren’t meant to be baby sitters or replace our roles in their lives. We still read together, eat together, learn together, laugh together, volunteer together, hang out together, but importantly: they hear the word “no”, frequently from my wife and I. Phones don’t say no.

The screen itself is not good for kids. This includes TV & has little to do with the content on it.

We unplugged our TV & held out as long as we could. It was when they got their first job that they got a phone & a prepaid SIM. They have always paid for their own mobile service but we bought them the initial phone.

I put filters on the router to stop certain content from being accessed. My 14 yo son bought a VPN service & got around it all.

There's no perfect solution. Just do your best.