Exactly, plus there's free, mostly unrestricted wifi everywhere. If your child has some pocket or birthday money they can freely spend, they can walk into an electronics store, buy a cheap smartphone or tablet and have unrestricted access.
At home measures are at best a delay, not a fix. What you also have to do is actually communicate with your child. If you're strict about what they can and cannot do on the internet, they will feel shame for doing it anyway, which may also mean they would be too ashamed to talk to their parents if for example they are getting groomed online.
That was originally going to be my plan - my kids can have a smartphone when they can afford to buy one themselves. I figured that by this point they would be old and experienced enough to deal with it. As I pointed out above, some of their peers at ages 5-7 already have parentally-supplied smartphones. It sucks that I'm probably going to have to talk to my currently 5-year-old girl very soon about what the internet has to offer.
At home measures are at best a delay, not a fix. What you also have to do is actually communicate with your child. If you're strict about what they can and cannot do on the internet, they will feel shame for doing it anyway, which may also mean they would be too ashamed to talk to their parents if for example they are getting groomed online.