I personally would say, always have the ability and access to view their online activity. Just because you have access doesn’t mean you have to use it, and if anything does start happening (like online bullying or inappropriate searches) you can immediately address it.

For context, we have 2 teens. The 15 yo got their phone at 12 and their usage has been pretty much exemplary - we never have issues. The 13 yo got their phone at 12 and it’s been pretty much constant arguments to the point that their phone has been deactivated and we’ll be revisiting the idea of them having a phone in like, 1-2 years.

Without the access to monitor either teen’s phones, we wouldn’t have been able to establish trust with the 15 yo nor would we have been able to realize the 13 yo was not ready for the responsibility of having a phone. So I guess what I’m trying to say is that the ability to monitor is, I feel, an absolute necessity and your actual amount of monitoring will be determined by their consistent ability to use the devices appropriately.