Reading comprehension skills fail
"A 20-year-old man and a 16-year-old youth were arrested near the scene."
That means the police arrested 2 people NEAR the scene of the crime, that, one hopes, fit the description of the attackers. It means that the police have NOT arrested 2 people caught in the act, or directly at the scene.
It could be the perpetrators, or it could easily be 2 people being in the wrong place at the wrong time who vaguely fit the description given by witnesses (e.g. 2 blokes, one tall one fat).
Given that that they've been given bail, it means that have a verifiable address, and probable were not covered in blood or such, otherwise the police would be bragging "we definitely got them". Police arrest the wrong people all the time, that's why we bother with court cases.
I really hope that the commenters with the knee jerk reaction don't work as admins/programmers/engineers, and are only managers or such...