These days, it’s used for emphasis to make sure you’re properly upset about it. Try using it in a context where age doesn’t matter, and you see how much implied severity it can add.
“I saw you speaking to a girl. What’s up with that?”
vs
“I saw you speaking to an underage girl. What’s up with that?”
These days, it’s used for emphasis to make sure you’re properly upset about it. Try using it in a context where age doesn’t matter, and you see how much implied severity it can add.
“I saw you speaking to a girl. What’s up with that?”
vs
“I saw you speaking to an underage girl. What’s up with that?”
One of them sounds much more like a crime.