Are they speeding? Then they are committing crimes, ergo they are criminals by the definition of the word.

Even if you ethically limit crime to harmful behavior, the risk to pedestrians and other traffic still paints them as criminals.

There are reasons they aren’t racing at a track, and part of it is the thrill of someone potentially getting hurt, or arrested. So even by your definition, they are criminals.

Now skateboarding is not a crime. Public nuisance, perhaps, at worst, but not a crime.