When they're old enough to pay for the internet themselves and deal with any reaction from others online as a result of their trolling.
It's not just if the memes/songs/comments are 'dark' offensive , it's whether they're actually illegal, in contravention of court orders or, as most kids/people aren't brilliant at keeping their identity completely hidden online, they could attract attention from very unpleasant people.
Are they psychologically able to handle the pile ons that some kids who have received due to expressing political comments? Thousands of notification a day, threats of violence, of rape, their names and addresses being made public? Being identified in the Press? Doorstepped by reporters or internet warriors?
I think and will say I think Yaxley-Lennon is a racist, violent and abusive prick. But I'd feel differently were I to open the door one morning and find him or his fan club there I'd think he was a racist, violent, abusive prick who was scaring the shit out of me by tracking me down and letting thousands know I, MitziK of number 28 Acacia Avenue, Bournemouth, alone at night and physically defenceless, disagreed with their opinion of their hero.
It all seems so safe to do or say things you think are funny in the safety of a bedroom in suburbia. But the internet is not a safe place - it invites the attention of unpleasant, violent, fanatical and at times, fucking demented, people. They aren't old enough to be able to cope with that or necessarily to protect themselves from it.
So until they're old enough to be responsible for their own bills and safety, I agree with your friend.