Because so many children are going outside instead of wasting their life on TikTok Twitch etc.

The amount of human hours spent watching others play video games / gamble / creating parasocial relationships: https://twitchtracker.com/statistics/watch-time

Curfew laws are quite common aren't they?

And cigarettes! What next booze!?

Libertarianism really does hit a wall when it comes to kids, in so many ways, doesn't it?

Not really, we just say the parents are more attuned to their child then the national government. I love the dystopian argument that without age laws parents would be out buying cigarettes and booze.