Isn’t it about responsibility and respect, rather than age.
Responsibility includes protection and welfare. I was POA for my mother with dementia. Step by step I had to intrude on her privacy to ensure she was not at risk. I took away her cheque book when she started giving her bank details to cold callers. I arranged overnight care when she was discharged from hospital. She did not like this and threw some major teenage tantrums.
My aim was for her to retain as much dignity and independence as possible whilst keeping her safe. There was a balance to had and that balance shifted according to circumstance.
If elected the same with DC. When DD was under 13 Irequired her to put her Fb account through my email address so I saw when she was tagged. So stuff was nasty and reflected low level school bullying, which gave me a chance to discuss this.
Beyond that I never had occasion to look at DCs social media. One DC was extremely cautious with a nice group of friends. The other tended towards over-sharing anyway.
However I would have if I felt a balance had shifted. If there had been a sudden change in their behaviour or friendship group. As a parent I would have wanted to understand what was going on. That responsibility would have trumped my respect for their privacy. But obviously done in a way to avoid building unnecessary barriers to future communication.
And yes, I think teenagers need and welcome boundaries, whatever they say. We have seen a few DC who received very liberal child centred parenting who have ended up rudderless, as if anxious about the responsibilities of becoming an adult. Weirdly some, given massive independence when they were young, have ended up as very dependent, as a result of their poor and short term choices around education, drugs and more.