There is never a situation where they are the criminal but an adult wouldn't be, as opposed to, for example, buying alcohol. So the harm falls on their lovers, not them.

But in most places, sex between two minors is illegal (edit: below a certain age, not just below the age of majority), and both are liable. And while court cases are rare, punishment at school and at home is common, and most teenagers have to vet their romantic partners with their parents.

More generally, and more importantly, minors have substantial privileges. People are legally obliged to care for them and ensure they have a certain standard of health. Schools are obliged to educate them for free.

Like Alicorn said, these are privileges that should be extended to adults. In many countries, a standard of free health care is guaranteed to all. In some countries, university education is free to all citizens. I'd like to see more of that.

Crimes against them are punished far more severely.

But this is only necessary because they're prevented from defending themselves the way adults would. Most crimes against young people (it's silly to call 16 year olds "children") are done by someone the law forces them to be in daily contact with, even if they hate that person. Such as parents and family and schoolyard bullies. (Most people aren't allowed to veto their K12 school, class, or teachers.)

Their own crimes a punished (usually) far less severely

If we're taking the US as a reference point, I would argue that adults should be punished a lot less for most crimes...