Exactly. Thankfully because he's only recently turned 10, he and his friends aren't old enough for their own 4G allowance. He's too young to be unsupervised at someone's house. But he's sharp as a tack, so I'd be amazed if he's not found a workaround.
We clamped right down when we heard about youtube videos with unsuitable scenes edited into the video, while the audo remained toddler friendly. There are also some unsuitable adverts in games, but you do what you can.
Ultimately, kids are kids, so he's likely to hear all sorts in the playground and outside. I'd be worried if they didn't try it on.